Hey dude,
I've gone from ATX -> mATX -> mITX -> mATX and now back to ATX again.
I also went the ITX road as I wanted something small and space saving. Although, I'm with barnsley on temperatures, that is a real pain in the arse to be honest.
When I was on ITX, temps were hard to control really, and they tend to get hot due to it's limited space.
The cases I have experience with though are the Corsair 250D (which you are considering) and the Cooler Master Elite 110.
From my experiences the Elite 110 is a cute little case, that take the least space you can get really. Although it does have it's restrictions, such as an SFX PSU would be best suited for it. It has limitations in terms of GPUs measurements and it can only hold an 120 AIO if you wish for watercooling.
The 250D is one of the more famous ITX cases out there, although it's quite big in my eyes, especailly since my previous experience with the Elite 110.
The 250D absolute worst and weakest point in my eyes are the cable management. It barely has ANY cable downs at all in the case and you basically need to tie all the PSU cables togheter in order to keep down on the floor itself. If you don't get like those doublesided cable hold downs and then use a zip tie around the cables on them.
It was a nightmare keeping the cables clean in that case and I spent numerous hours with it and frustration was really building up, untill I realized that no matter how much I tried, it wouldn't become good at all. The case just has too bad of a cable management design in it, unfortunately.
It does support an H100i in it, which is one of it's stronger points and you CAN fit an 200m fan in the front, but it's tight and only work with certain fans of that size (I've tested).
And with an 200m fan in there, the airflow was quite nice indeed. My mom has the 250D now after I built her an PC of older parts that I had. And I gotta say, aesthetically, it's one of the builds that I'm most proud of.
In my eyes, the 250D is a bit of a hard choice really... I know the feeling when you want to get an ITX case and the 250D looks really promising. But when you've worked it in, you'll get a second opinion on it completely, as it's not really build friendly in terms of cable management, which is a huge factor in terms of overally airflow and temperatures in ITX builds.
Hope this helps a bit, I'm aware that it's not much positive thoughts etc for you, but I rather be honest from my personal point of view, rather than not being. For you to be aware of my experiences with it, to keep these things in mind. That it can be quite tricky to work with, when you make your decision of which case.
Any other questions you might have, let me know and I'll try to help you out the best I can
