Basic difference between 4.1 and 5.1 is, in a 4.1 you have 4 corner speaker and a sub. In 5.1 you get a additional center speaker.
It depends on how you like the sound. I mean in 4.1 the speech would be distributed to all 4 corner speakers but in 5.1 the speech will be relayed to center speaker. Because of this reason few prefer 4.1
Better have a demo at a retail shop to have the feel. During demo use all sounds like speech scenes, music, action etc only then you get an idea
And one more thing dont get stumped by the speaker output like 4000w or 5000w ask for the rms watts. And check whether it supports all latest sound formats like dolby digital, dolby pro, dsx, dts, hdts and few more (not sure of those names)