Differnce

Whats the difference between MF2 and GF2?

There's a comparison chart here:

http://www.clickteam.com/eng/compareproducts.php

One of the most important differences is MMF2's ability to use custom-made extensions - these really open up a lot more possibilities. Other than that, executables built using MMF2 don't have a compulsory logo at the end, and can be sold if a smaller, less intrusive logo is included in your game.

About $60 and the ability to use third party extensions which is only offered for MMF2 and MMF2 Developer. TGF2 also includes a little splash logo for TGF2 at the end of an application. There isn't much major difference other than that, that I can think of.

---EDIT---
Beaten to it! :)

There are also some Clickteam provided extension that exist for MMF2 but not TGF2.

Also MMF2 support more sound and graphics format then TGF2 does.
Most notable would be OGG sound file support.

The compare link above gives the best info
http://www.clickteam.com/eng/compareproducts.php

MMF2 supports alpha channel.