Being the total (wheel and tire) diameter for the 19" and 21" is going to be pretty close, the contact patch should be the same (front to back). Only difference would be if the 21" wheels/tires are wider (giving more contact area and better acceleration grip). But, then, of course, tire specs are typically geared more towards performace (softer compounds and such) for higher diameter wheels. So, with all that, I'd say you'll technically get better acceleration with the 21" wheels with more performance spec'd tires, but whether you'll really notice it is to be seen. Finally, acceleration specs is also alot about the driver, so that can be a factor too, unless traction control is involved where all you have to do is punch it.