I have a few 25x scopes(Razor, TT) and am interested in a 35 or 45 x scope so I can use as a spotter also.
Does a 5-25x set at 25x get affected more by mirage then a 35x scope set at 25?
It gets pretty hot in Texas. Does a spotting scope do better with mirage due to its size?
To answer your first question, no. If both optics are set at 25X, they will be affected by mirage in a similar way.
To answer your second question, the impact of the mirage is going to be influenced optically by two aspects; depth of field and magnification. It so happens that DoF and magnification are related. But to your point, if the spotting scope and the riflescope are set at the same magnification the mirage will affect them in a similar way. See answer 1 above.
Of course, it depends on how you use the optics and what your goal is. Let me explain. I'm an F-class guy and I compete year round, mostly in south Texas. I use a spotting scope and riflescope when firing individually. My spotting scope is a 27X fixed optic with a long eye relief eyepiece and my riflescope is a 5-50X56 optics set usually at 40X. I sometimes crank it to 50X but it's mostly at 40X, all the time.
I focus my spotting scope to about 650 yards downrange and thus provide me with about the deepest DoF possible that is also useful. This means I get the mirage from about 400-450 yards all the way to the target line; I want to observe as much mirage as possible. On the other hand, my riflescope is focused exactly on the target at 1000 yards, and I would have to go to a DoF calculator but I would be surprised if the DoF starts any closer that 900+ yards downrange. In my riflescope I only observe the mirage at the target.
Does that mean that my riflescope is unaffected by the heat waves from 0 to 900 yards? No, but they don't show up as a river like they do closer to the target on in my spotting scope.
