Hm, was a move to 3d models in Gettysburg already decided and announced? Or maybe it's for a future game.
If it's already done you probably shouldn't read this. It's your game, this is just my opinion.
I think it's a bad idea myself. A blend of 2d sprites and 3d models is very hard do well, at least well enough to be better than straight sprites.
Inconsistency is the main problem. A game can have very detailed graphics, but if the type of detail is inconsistent, the whole thing feels fake. TC2M didn't have as much detail as say, M2TW, but the scene feels much more real because the graphics, while simple, are beautifully consistent. Nothing looks out of place. The sprites that are of course necessary in M2TW look entirely out of place. You've got some detailed graphics up close mixed with ugly sprites and it ruins the feel. Consistency is more important than overall detail.
Behold M2TW with high graphics settings:

(I don't have many M2TW screenshots left having not run it in a year. The forests are something that ought to be seen, they are ruined by the sudden change from 3d model to flat sprite. If they were all flat sprites it wouldn't be noticeable.)
Sprites also generally animate better from what I've seen. Most war games that use models have animations that look completely fake. TC2M sprites did look like they were doing the real thing, not floating in a lethargic manner and suddenly jerking around.
I'm sure you're well aware of the vast potential for performance issues with models. You're smart enough to figure out that the number of models displayed should be constant, not dependent on how many objects are close enough to the camera. Making it constant prevents some of the problems.
The last problem with using models is that you can't have the best of both. If you use models, the sprites are not going to be as good as they were.
Eventually it will be possible to do it all well with models, but we're not there yet, and I don't think we will be for a long time.
If it was possible to use 3d models without losing the detail and consistency of the sprites it wouldn't be bad, but I've never seen it done.
Finally, and unfortunately, it is true that you can get screenshots and videos to look amazing with 3d models, as they did with M2TW by disabling the sprites, and it will directly increase sales. Because of this I won't blame anyone who decides on models, business is business.
It lead to a lot of disgust with M2TW (and now ETW) because the sprite/model inconsistency is so glaring.