I've had both, the Proto is a little lighter, a little smaller, not quite as easy on paint, but more gas efficient than the DM4.
The DM4 is a little bit quiter, runs at slightly lower pressures, looks awesome compared to the Proto.
Both have the same trigger, and the software on the boards are not radically different from one another.
As for Protos being easier to maintain, well, with the exception of the DM4 on/off at the front, day to day maintainance is exactly the same. The DM4 bolt may be a bit longer than a Proto one, but the lubrication of the o-rings is exactly the same.
I would pick the DM4 for looks and slightly softer shooting characteristics.