Both have almost the same engine. Almost everybody knows this, traxxas fan or not.
Acutally, both have the EXACT same engine. I know that and I own a RC10GT.
What does the 2.5 mean then
The 2.5 is the displacement in cubic centimeters (cc's) a .15 engine is 0.15 cubic inches (cu. in.) or 2.5cc's I guess Traxxas used the 2.5cc to make the engine sound more powerful than a .15, even though they are the same size. The "Traxxas TRX 2.5" is just a .15 engine...