If you're very interested in a specific trainer and feel the need to have it in the sim, then there is a clear answer to that.

G3.5 and FSOne are both biased to each's product line. You'll see Great Planes models in G3.5 and not a Hangar9 to be seen. And vice versa with FSOne.

The truth is, with trainers, none of them fly uniquely enough that it would benefit to have it simulated anywhere.

I'd suggest either of the two most popular simes would be an excellent choice. G3.5 is constantly offering new models. They cost you, but so do the real ones. And it insures that G3.5 stays around in the future. So if new models matter, of the two most popular sims, there is a clear choice.

Beyond that, there are fanboys of every sim. And almost all the sims are good to have on their own. But don't base your purchase on the trainer you might want. There really isn't any trainer that is head and shoulders above the pack. Darned if most of them fly almost exactly like all the rest, in spite of what their advertising says.