Well there's a little more info to give then just a little different compression. The LQ9 is 10.1 to 10.4 depending on what source you read. Also all LQ9s will ALL come w/ 317 heads (similar as the 243, but with larger 76cc chambers) (243 heads are the heads they put on the 405hp Z06)
Not all LQ4 come w the 317 heads and the comp is only 9.4:1 like a 5.3l.
Either motor will be bad ass in your truck!! A lq9 is just like a 40hp bonus. SUPER CHEVY shows LQ4 w/ 317 heads at 360hp (engine dyno I'm sure) and the LQ9 at 400hp. Read up on it, a lot of good info. Like little things that make them different like having floating pins and stuff like that.
BUT again LQ4 or LQ9 will both be bad ass to have!!!! a lq9 is just like getting a little bonus when you buy.
SUPER CHEVY/ All LSx motors info
http://www.superchevy.com/technical/engines_drivetrain/sucp_0911_ls_engine_performance/viewall.html