Well, the first question would be, since the SuperNait has been around for a few good years now, whether you have ever had the chance to demo one.
It's not a bad machine and it's very good value, particularly on the SH market, you have several upgrade options but the sound signature divides opinions. Quite a few owners of higher end Naim stuff will tell you that it's rubbish, with a mellow, bloated sound, unacceptably high hiss and hum levels, a below average transparency and detail retrieval. On the other hand it has quite a bit of power (although it could never drive my Ultimatum's to acceptable levels unless biamped), the warmish sound signature is easy to listen to and quite pleasing while it still retains most of the typical Naim qualities. So it is not without fans, in fact I know quite a few people who prefer it to the NAC202 / NAP200 combo.
Now SuperUniti shares a lot of the Nait's qualities and has a similar sound signature but it's a bit leaner, more hifi, detailed, less bloated. Which sounds very well in theory, it's just that it's not as energetic, engaging, lively and warm as the SuperNait. And some things tend to sound a bit artificial and digital through it. Just a bit, it's not a huge difference but I think the SuperNait does voices, live recordings, drums, rock and similar better while the SuperUniti might be a better choice for classical, acoustic, that sort of stuff. I would certainly advise you to try them side by side because most of the people I know prefer the SuperNait.
Besides there is the question of whether you want to stop here or plan to upgrade in the future. You can get a HiCap for the SuperNait which later could be used to power a NAC202 / 282. The SuperUniti doesn't have any upgrade path, apart from the external amplifire which kind of defeats the purpose of having a one box solution. Better still a 272 / 155 or whatever can be upgraded step by step, allowing you to spread the costs and enjoy the upgrades step by step.
And while the SuperUniti has a better dac than SuperNait, no question about it, the standalone DAC is a big upgrade over the SuperUniti. Combine it with the slightly more pleasing, fuller and livelier sound of the SuperNait (and I felt a bit more powerful as well but I might be wrong) and I think you have a better proposition. The DAC, don't forget, is an upgrade path for anything below NDS and it is in my opinion one of the big bargains in the Naim range. And a HiCap for the SuperNait and a XPS / 555 for the DAC will widen the gap.
But in the end it depends on what exactly you are trying to achieve. If you have little space and you simply want something that sounds good straight out of the box, no snake oils, cables, racks, power supplies - SuperUniti delivers, although I would certainly try to give it a listen first.
If on the other hand you have plans for the future and this is just a stage in your quest for building a proper hifi system, I think there's more value to be had elsewhere.