sx is louder than R at rated powers.. but get up to 1100-1400 + rms and many people will say that the R can wang with the sxs no problem.
tommyk did a direct comparo with type r, sx, sexxx, and the sx was quietest in a couple different installs iirc...
BUT regardless... i think he should get a single 15, whether its RL-p, SX, or type R, all will sound pretty decent, all will get pretty loud, and its up to him whether he wants what mix of sq or spl, and how much power he can run with a stock elec.
PS fryman im still waiting for that dollar you owe me //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif