I have thought about this particular scenario a few times.
I think, in an all out battle, General Rieekan would eventually win. It depends on how he positions his forces. The MPTLs and AT-TEs will be useful in taking out some of the heavy walkers, and the specforce troops will be able to take on the Snowtroopers for a time, but I feel that Rieekan will be pushed back into Echo Base eventually.
But this is actually a good thing since there will be no retreat (if there is no option). General Rieekan can position his troops and set up traps for the Snowtroopers (who will be cut off from the AT-ATs). So once the supporting Snowtroopers are down, the SpecForce soldiers and whatever reserve armor Rieekan has can be moved (from what was the shuttle landing zone of Echo Base) to flank the remaining forces in Blizzard Force.
All in all, I think that Rieekan's position in Echo Base (and the fact that he doesn't have to worry about retreating to save the Rebellion) will win him the battle. The Heavy Armor helps too.
Was thinking along the same lines, truthfully it seemed the Rebels only really retreated due to a lack of Armor amongst other things. Have no doubt, if they have some vehicles with them the battle would have been more in their favor provided they would be able to quickly down the AT-ATs.