I have the Extra also... It's pulled around by a saito 91... Needed some nose weight. Flies insanely fast. Thing is the Extra was a .40 A/C that they "upgraded" to a 60 class... And it doesn't look like they made it bigger... Hell my manual still read all the 40 size engines as choices! The 540, I believe, is actually built as a real 60, not an upgraded 40.

The wings show the same span BUT the weights show 6-7 on the Extra and 8-9 on the 540. The fuse in the 540 is also larger than the extra. That 2 lbs is the difference on the engine. The wing loading goes up by about a few ounces.

The extra you can put two servos on the elevator which can make for some fun 3D if you want. I'm not sure about the 540.

All that said I think the 300 for the 75.... If you had something bigger around that would be ideal for the 540.