Well, I've been gone for quite a while, but I'd like to weigh in on some of this discussion, as sniper rifles and DMR's are my favorite subject in airsoft.
A good, upgraded Spring rifle has certain advantages over a DMR:
1: Lower cost to upgrade, and generally easier to upgrade/disassemble.
2: Very Quiet
3: No batteries/Gas to worry about.
4: Topped out, these rifles have the greatest potential as far as distance/accuracy goes.
(Disclaimer: As with ANY weapon, the performance of your weapon will be dependent on how well you take care of it, and how good a job you do with your installation of upgrades.)
Now, a DMR has certain advantages too:
1: Higher rate of fire.
Well, that's really the only Pro, now that I think about it.....
as far as DMR's go, They can provide more rounds on target quicker, but suffer from the whine of a mechbox, reliance on a battery, and a much steeper price increase on top end upgrades, and require more technical knowledge to maintain the more complex mechbox.