Have to agree with tony.
In our club there are 4-5 guys that run MO4's. We use RP 36r's for the most part and the cars compete well with the M03's. Top 5 for this past season in Mini went 1st M04M, 2nd M03M, 3rd M04M, 4th M03M and 5th M03M.
I run an M04M and yes it does have a slight advantage on the top end. For me the M04M feels easier to drive smooth. But it can be touchy especially in the steering if the saver does'nt hold. I have had the car start a race tracking straight, then hit a rail and the car will pull to one side, then hit a rail again and it's back tracking straight!hehe
I started out in Mini last year with an M03M and really enjoyed it and if not for getting a deal on my M04M ($40) I probably would still be running it. To me the M03M felt like I had to drive it on the edge or push it hard to get the most out of it.
I think both cars run good and may work better in different situations. But if everyone else is running M03's maybe find out if anyone has run an M04 and see what they think about it.
For our club because we don't have to run Tamiya tires this really seemed to level the playing field between the M03 and M04 and maybe a slight edge to the M04
If your going to buy both cars anyways then you can't go wrong either way.