Antec P280 has more room, provides noise dampers and good ventilation!
Even better, it has 270-degree door that can swing right against the side and provides dust filters.
Second, this case has screwless retention mechanism and vibration reduction (rubber grommets) for your hard drive / ssd.
I own the Define XL and it's by far the best case I've had. The sound insulation is good and the overall build quality is great too. It has a distinctive, sophisticated look.
Well I tossed a coin and came up with the R3...picked it up for $115 from NCIX...only $10 cheaper than the P280 on Amazon. I'm only running one GPU and couldn't justify the much larger footprint of the P280. I also like how the front panel ports are on top of the case instead of on the front since my tower will be on the ground. Either way, I don't think you could go wrong with either case.