Both are fairly easy to maintain, but the Geo has a small edge on maintenance. I tend to lube my 3.5 after every 10 cases, but could probably leave it far longer.
At the end of the day, both markers shoot paintball out of the barrel. I'd personally recommend the Geo 3.5 because it is rock-solid reliable. But on the flipside, I've not owned a DM15. I've got a friend with a DM14 (basically the same) and he's only had one issue with it.