Biggest difference is the M2 is a twin scroll single turbo vs the M3’s actual twin turbo.
The M2 basically uses a tuned version of the older M235 engine, but borrows the forged internals of the M3. I believe they both use the same rear exhaust.
The F80 M cars, especially the comp pack versions, are criminally underrated hp wise and are roll race killers. Bone stock they easily trap 120+ and will put a hurting on a lot of machines.
The M2 is also a bit underrated and is no slouch in its own right but it’s giving up arguably 100+ rwhp with only 100 lbs weight advantage. No match for its big brother.