AG 300 and M-Fil 300 are very similar in many ways...both have a very solid feel that is surprising for their light weight, both work well as an all-court racquet, volley well, provide great control, easy access to spin (I get more from the MF3), and the MF3 serves really well, AG just a shade behind in terms of control/placement.
The AG3 has more of the following: Power (the difference seems to really increase as you drop tension in the AG3); "thumpy" feel (AG3 has an even more thumpy/solid feel than the MF3, but is also has a bit more muted feel); solid volleys (AG3 a bit better here than the MF3). The AG3 still has very good control even w/the additional power. The only down-side I've found w/the AG3 is that it doesn't quite maneuver the way you'd expect it to based on specs...feels less maneuverable than the M-Fil, rather than more, which is strange given the AG3 is spec'd more head light.
Either would be a good choice, but I would certainly demo if at all possible before buying. The MF3 is going to ask you to generate more of the power, it's low to moderate power but more on the low side, the AG3 is low to moderate, more on the moderate side.
I haven't hit the AG200 or MF200 (except the MF200 2006, which doesn't appear to be available any more).