With Launcher Pro Plus you get a bunch of awesome widgets plus they now have popup notifications you can put on the dock for new texts or mail or things like that.
Start out with LP, because its free and the other is just a key.
As best I can tell the only benefit of plus is some social media widget (come on, is there really a shortage of those) and the ability to stretch some widgets if they don't offer the size you want.
Has anyone detected any performance deterioration from using LP+? Or problems with it? I've had it a couple times, but went back to the stock because Moto widgets don't do well with LP+, even on 2.2.