i'd go with the biggest engine possible - so yeah, the A15.
these little babies need all the help they can get in the "low down torque" department, and the extra rev-ability of the samller engine will be offset by the lower amount of torque it produces. this might make it a great power producer for it size, but be a pain in the proverbial to 'drive' the thing out of corners or get any decent speed out of it.
that's my happy meal with fries anyways...