I was VERY impressed with a Mueller Sport Dot, but I didn't have a chance to shoot with it; just fondle. Extremely simple reticle but it flat-out worked.
Combined with decent turrets and a cover mounted dop-chat it should make hits at 600m totally possible. Then again I've never been a fan of BDC reticles, I figure if you've got time to count dots you could just as easily dial it in and aim directly.
Personally I'd pass on the DMS. Shot a few rounds with one at the range. Clicks were poor at best, not terribly repeatable either.
Of course a RazorHD, SWFA or Sightron 1-7 would be pretty tight if the budget allows.
-Grant