Well... that's the kind of things you should discuss with a proper A/V dealer that's gonna go to your home and see the best fit available. For that kind of purchase, don't worry, they're gonna bend themselves in four to please you... And any decent dealer is not gonna try and rip you more money than necessary, because you're already planning on spending quite a lot. Of course, a decent dealer in which you can trust might not be so easy to find... so a good thing, in any case, would be to seem to know where you're heading, at the very least. Maybe I can help you with that.
Don't know much about Onkyo as I don't sell their products, but for what I saw on the net they seem to make pretty much decent A/V receivers, just like Pioneer does, and looking quite alike too when seen from the rear. If Onkyo is even better than Pioneer at getting a "good" stereo sound out of that pile of EMI emitting components, then hey! just run with them. Power is not a problem with much of both their high end receivers, as Pioneer at least will rock your world at very decent volume with any decent speaker... but for music, fidelity might be. I know that Pioneer offers on all of their Elite receivers an I-connection that allows the amp to take the information straight out of the I-device memory, allowing a better music sampling... also they have Sound Air Retriever, which is supposed to help "expanding" the compressed data. Developped with Air studio, and exclusive of the royalties they're paying to Apple. You'll hear about it when you'll shop, if not already... but if you could actually try it, it would be better, as for what I could hear it doesn't make much of a difference anyway... Arcam Mini Solo I-Pod streaming sounds quite as good in it's way with Monitor Audio Radius 90 and 360 Subwoofer, than Pionner with Paradigm Cinema CT70 and sub (played in stereo) sound in their way. Differences are mainly due to the companies, not the streaming. The sound does change when you activate the SRA feature on Pioneer, but is it really better? The said connecting wire included with the latest Pioneers (except VSX821 and under) looks so cheap, probably even if it somehow took the data straight out of a SACD it would still sound like compressed music. And one of the "problem" with Hi-Fi, to my sense at least, is that it doesn't help covering up the compression damage, it can only enable it. Pioneer they're also saying they improved the Stereo settings of the amp... but I'm not so sure I hear that. Pretty sure VSX521K sounds pretty much the same than said improved SC-57 when in stereo mode, unless for power of course. Not that bad for the average listener, but... Nothing like a stereo integrated amp, or even a good Ampli/Tuner receiver, to at least start getting something out of music. But as I said, if Onkyo A/V do have a better sound when played in stereo, then it's worth considering an A/V amp for universal use. Arcam released a Blu Ray player that reads pretty much anything, even SACD, and internal specifications sound good stereo wise for such a universal device. But the Arcam entry level A/V receiver I heard is not quite special, so perhaps you should keep with "cheaper" A/V dedicated companies... like Pioneer. Cause when it comes to surround and HT, Pioneer Elite does deliver, and the latest THX Ultra is seriously mad. Still, as you don't intend to actually install a 5.1 for you don't have much room, any A/V receiver won't help you : sure they got multi-zone, but don't forget that 5 out of 7 or 9 channels are necessarily dedicated to the same zone... you can wire the 5 channels zone into three different rooms (example : two floorstanders in each of the two first rooms including cinema room, and a stereo single speaker in the bathroom). But bear in mind that controlling those three rooms will be on a single zone... so the sound of the movie you're listening is gonna play in those two other rooms, and if any person wants to listen to something else in the said rooms (like your other half taking a bath, for example), they won't be able to. That was the very idea of a multizone A/V amp : that the first zone (at least) would be for only one room, dedicated for cinema. Pionner Elite 9.1 offer a two zone simultaneous control out of three, so the third zone (if you wire only a basic 5.1 cinema system) is a fake, really. For example, you're listening and controlling your movie in zone 1 in 5.1; you're other half is listening and controlling music in the bathroom (through I-device using Airplay and router); Tuner was set in the third zone of a third party to enjoy... but not to control in any way, tuning or even volume. And switching that so as the third party gets control implies depriving the other plus-zone of that control... I don't know how it's made, as we use them at work for cinema multichannels entertainement only (though controlled through Airplay to show the possibilities), but I'm pretty sure it would imply that even the first party controlling the main 5.1 zone would be disrupted as well... In any way, if you decide to use the three zones on a 9.1 receiver, you will necessarily encounter controlling problems if you're more than two persons using the same receiver... and sooner or later you'll regret your choice. And as you don't intend to actually use the whole basic 5.1, then I don't see the use at all, even if you're only two persons... If you're alone, then it can be done up to 5 rooms. But don't forget : device played for any of the rooms connected to the main 5.1 zone will actually play in all these rooms. Ain't much of a solution, is it?
What you really need is a careful planning of the wanted connectivity and use, and some decent, first-person on the spot advice as how to achieve it and get a maximum of enjoyment and easy control.
Please, don't ever buy a sound bar, unless one exists that really outruns the ones I know. I've heard some pretty expensive ones (covering the buying of decent starting speakers and amp), and pretty cheaper ones, and I wasn't impressed by any of them. Still, you'll need to hear some for youself... Any good 2.1 in your cinema room is gonna get you all the enjoyment you need in a small space, movie, gaming, and music. A sound bar is often a bad 2.1 connected to a bad amp/sub with bad angled speakers seemingly reproducing nonexisting surround sound by throwing the same much-too-tweeter sound in every direction it can achieve from it's ill-conceived display. One other thing I don't like about them, if they really do achieve some sense of surround instead of just beaming in some four directions, but your room doesn't match very well with those sound waves... can't much move or angle your speakers differently, can you?
One other thing... perhaps if you post the same question in the dedicated HT forum, you'll get more advanced answers on a couple of points I tried to cover here.
Hope I helped some!