I own both of these cases. I'd say that the R3 has better cooling potential than the XL. Xbitlabs did a review and found that the GPU cooling was better in the XL than in the R3, while the CPU cooling was not. One of the advantages of the R3 is that it has a place to put a cooling fan on the bottom, whereas the hard drive cage in the XL prevents this. I had to install the optional 5.25" bay 120mm fan in order to improve my CPU cooling. I think the case is so big that it's needed. I'm unconvinced that the 180mm fan in the top is a great idea either, with its restricted output vent. If that vent were not right near the rear case fan exhaust I would certainly consider turning it around so that it acts as an intake fan instead. This is an option available to you with the R3. Also, you will definitely need to add more intake fans on the R3 to create a positive case pressure. The combination of the top exhaust fan and the rear case fan creates a CFM of somewhere in the region of 150 (depending on the RPM of both fans - I run my rear case fan at 900 RPM), meaning that the single front intake fan on the XL is insufficient. I balanced it with two NF-SB12FLX at 900 RPM and a NF-P12 in the 5.25" bay at 900rpm that together give me a CFM that slightly exceeds that exhaust fan CFM and delivers cool air more directly to the CPU heatsink. Don't underestimate this - orginally I had a NF-SB12 ULN in the 5.25" bay and swapping it for an NF-P12 resulted in CPU temps three or four degrees cooler.
Generally, I think the R3 is a slightly more flexible case, although I'm very happy with the XL (I have all 10 drive bays in the XL populated along with two 3-slot-width graphics cards). It's cable management possibilities are really good, and tere's tons of space to work. Make no mistake - it's a big case. I don't think you can go wrong with either, but if you don't need the size and the 10 drive bays in the XL, I would go with the R3.