I know my Arcam is much more expensive than the 400, but I have owned a 350 as well before, so am qualified to answer the question posted.
I have one room for both Hi Fi and home cinema. it is an ideal shape and layout for 5.1 cinema speaker placings and has room for a sub. It would be a nightmare to wire up the same speakers to two amps I didnt want to have two many boxes. If you want two systems, you would need at least 6 boxes if you include a Playstation and maybe more if you want a seperate CD player and Blu ray/DVD player. I decided years ago that it could only be one system, hence my kit choice shown below.
If you choose seperate systems you will not be able to spend as much on each as you would like so that would be a compromise anyway unless you have unlimited money.
I am extremely happy with my combined setup but it depends on how much you want the "ultimate Hi FI" as to which way to go . My HI Fi nut friend will only buy Naim and has a hugely expensive set up just for that and a cheap cinema system which he is not that bothered about be he loves my setup. Its loud and full of impact( makes the wife scream sometimes) but also sounds really good on cd's and particularly on SACD from the OPPO( brilliant multi use machine)
I had initial firmware problems with my 600( very similar to the 400 I think) but I can honestly say it has now settled down with the updates and causes me no grief. No volume control is no problem, I just sit and squirt the remote, I have never missed it at all.
Go and listen would be my advice, I would be very surprised if you come away with two systems.I would not change mine now even if I had a bigger budget.