There are a lot of diff in the subs if you go to REAudio.com you can see. A few off the top of my head is the RMS power input of the sub and the Xmax of the sub. SX is a killer sub. States a 1000rms but I do 1500 from a PPI mono block to the one in my Kompressor. It has a good amount more Xmax too meaning you will have a single sub move alot more air giving it a louder and deeper sound with more ease. I sell RE so if you need more help let me know. If you are looking to do a pair or single one of each SX will be the best but if you want 2 SE 12 Vs. 1 SX 12 then the 2 will be the way to go since you can move more air that way. You then have to figure out how much room you have to work with.
Have to say no matter what you pick the sound will be sweet. Just how much sound is up to you!