I just ordered a TC Encore rifle in 25-06 and a Sightron S2 scope.

My choice of mounts are Leupold (steel base and rings) or Weaver (aluminum base and steel rings).

The dealer told me that the Leupold bases are more solid than the Weaver bases, but the Weaver rings tend to hold better than the Leupold rings.

Which are the better mounts?

Either; the .25-06 ain't going to kick hard enough to giggle things loose.

The T/C Maxima bases are steel, and work with Weaver-style rings, either Weavers or T/C Maximas, too.

Depends on which Leupold bases and rings you use. The so called standard rings and bases with the windage adjustment are no where near as good at hanging on to the scope rings as the Weaver set up you describe.
JimF, for instance, uses the Aluminium bases and Weaver style rings on many of his super light back packing rifles and has never had a problem with them.
I've seen the so called "better" all steel standard, windage adjustable rings and bases separate in the field. It's not hard to understand why. The rear ring has very little to hold on to.
On the other hand, Leupolds Dual Dovetail set up is as tough as they come. E