after researching all day today, i noticed a lot of .12 are more powerful (hp wise) than the .15's. but companies reccomend the .15 for offroad use, and .12 fo on-road. my guess is the .15's are based on torque, such as a real truck meant for towing is.
check out novarossi.com to compare the specs on these. it's very easy to at this site, which is why i reccomended it, not because i favor them or think they have the best engines...