I used Panaracer Pasela TourGuard 27 x 1 1/4 on my Univega Gran Turismo for a number of years and never had a flat. They have low rolling resistance, cushy ride, enough tread to keep you feeling stable in the rain, and not super heavy. And they're pretty cheap. I think my rear tire went about 1,000 miles before showing any signs of wear and tread loss (albeit minimal tread loss), and the front showed negligible wear. A few hundred of those miles were loaded touring miles. Most of them were semi-loaded commuting miles (rear panniers only or rear panniers + trunk bag). I also did the Seattle to Portland ride using them. They also handled well on dirt roads and okay on gravel, but a little extra caution was needed on gravel due to their width.

I think they changed the name of TourGuard to something else. It's their name for flat protection, but having inspected and used both Pasela TG and regular Pasela side by side, I'm convinced they're exactly the same. Give'em a try. If they're not to your liking, you're only out like $20.

EDIT: Also, Schwalbe Marathon Pluses are totally overkill. They are heavy, thick, hard rubber. They are nearly indestructible, but they're also one of the most uncomfortable, sluggish rides out there. The standard Schwalbe Marathons are equally indestructible and slightly less sluggish but still overkill. If you want to go that route, a comparable tire to Schwalbe Marathon is the Continental Contact. They're a faster, lighter tire but still have great flat protection. I've had good luck with them. And just so people know I'm not unfairly knocking the Marathon, I've got a pair of 26 x 2" Marathons on my LHT right now. They're good for heavily loaded, off-the-beaten-path touring, but they're probably not what you're looking for.