If I had only those two choices (for whatever reason), the 250D would be the one.
Rosewill makes a decent case and we both know Corsair's quality but three features of the 250D catch my eye - front USB location, drive access and bottom clearance. Of course, with the PSU on top in the Rosewill, the 1/2" bottom clearance is enough but I like that 1.5" of the 250D. I don't care for the bottom USB location on the U3 but if on a desk or shelf, it could work for me. Depends on the usage of them, I guess.
If I was to put the box on the desk, I would probably think of the windowed Rosewill with the thinner, taller dimensions but if on the floor (or low), the 250D with that cool top window. The 250D also gives the option of an optical drive which I still use.
The Rosewill has a smooth look to it. They're both 'pretty' but the filtering of the Corsair always wins me over and, for me, it looks better constructed as far as placement of things. All thumbscrewed, full access and tool-less is another plus for me with the 250D. In fact, I just put it in a Wish List for future reference. I like it!
The reviews on both are telling for me but I find the 250D has more appeal for my needs and expectations. Good luck with your choice - they both look to be pretty good options.