I tend to find the often misplaced truth about the DMs and reliability to be perpetuated by newer players who haven't actually ran a DM for any length of time, or by those who have used cheap O rings to fix a barrel leak issue.
I've been shooting all sorts of guns and have never found any DM any harder to maintain than a Geo or any less reliable. (Context of both guns owned: DM 4,5,6,7,8,9,10,12/ Geo 1,1+,2.0,2.1,3.0,3.1) ninja edit-ive also used the DM14 for a bit and I've had a good play with an M2 as well and to be honest it's hard to surpass the M2 in terms of feel (much like a rare working NT10)
With all markers if you want it to run at optimum efficiency you should look after it, all that both require is a quick lube of the bolt and you are done.
As for pros and cons of each, to be honest they are very very similar, I find the Geo louder and not as smooth but the build quality feels higher, the DM feels smoother to shoot and quieter but the trigger feels cheaper than the Geo counterpart. Both guns will serve as excellent guns to shoot people in the face.
My advice would be buy a shocker