Honestly, the 2 speed is just fine. The 3 speed is a bit of a pain to get it to shift right and even when it does, the lower/higher gearing it adds isn't really noticeable. I guess it depends on the engine you have. If you have a strong engine, then you probably won't benefit from it much. If you have a weak engine, it will spread the load more throughout the gears, so it helps with power somewhat.
I'm on the verge of buying a two speed on ebay because mine came with a 3-speed and it's more of a nuisance than it's worth.
I'd spend your $ elsewhere.