With the AC Commander under TMCC the GP7s will run consistently in the 20-30 scale MPH range on 054 curves pulling a train without complaint. I installed diecast fuel tanks under both, and the added weight helps a bit as well.
The best choice partly depends on your operating style. If you just want to turn the train on and watch it go around, an MTH PS1 with two can motors wired in series will maintain 10-20 mph all day under typical layout conditions, with minimal variation. All can-motored locos can be upgraded with speed control, which would eliminate any variation whatsoever. Personally I find this a bit robotic, and even boring. However, if you're a hands-on type operator who is more inclined to stop, reverse, switch, etc., it's the way that locos with dual Pullmor motors accelerate a heavy consist BELOW 10 MPH that makes them more gratifying to operate.
My other point is that locos with rubber tires on opposite sides of the same axle slow noticeably on very sharp curves. Speed control has to work very hard to prevent this. Lionel always put both of its tires on the same side of the loco, which gives up a little traction, but promotes smoother operation.
I think a member recently offered die-cast fuel tanks for sale on the Buy-Sell forum. If you can't find them, don't worry. Adding a couple of lead tire weights inside the stock plastic fuel tank works just as well. Fun times ahead!!