I put an UltraDot on my 678GC. Love it and recommend it highly.
If you get a 30MM UltraDot, call Ruger and swap the 1" rings that come with the gun for the 30MM ones. Ruger will charge you $5 for S&H and they'll be in your mailbox in a couple days. The very nice lady on the phone suggested I use the large/longer base for the UltraDot, but I thought the standard one would work fine (it did), so I gave them a credit card and they sent the long base so I could try both. My card was refunded very quickly when I sent the rings and unused base back.
Sturm, Ruger & Co., Inc.
200 Ruger Road
Prescott, AZ 86301
Telephone: 928-778-6555
I really like the Ruger mount; I'm able to remove the dot with a Quarter and shoot open sights and then remount the dot and the zero stays nearly dead-nuts (maybe off by a click at 25 yards).