depends on your application.

Many like to stick with the ease an security of the stock 2T pump. Even for racing.

Me, I like to premix, as my bikes only ever get used for serious track hooning these days.

If you're riding on the street then the 2T pump is the way to go. Premix will smoke like a bugger at lower rpm, and you just don't need a 30:1 ratio when you're cruising down to the chippy to try and impress the girls.