Lq9 has higher compression (flat top pistons) and only likes high octane fuel. The LQ9 can be found under the hoods of Vmax's and SSS's.

The LQ4 is an almost identical truck motor with lower compression (it has dished pistons). Its used in 2500's and will run on low octane. Imo, unless you are going to run a considerable cam the LQ4 is the best bet.