You wanted advice selecting a 2nd 3D plane, but the choices you listed aren't really 3D planes at all. So if restricted to those choices, then you might as well choose one by flipping a coin.

The Harrier 90 that I flew had a Saito 100 in the nose. Even with that engine it was marginally powered for 3D. I would suspect the 75AX would barely be able to 3D a Harrier. Plus IMHO the Harrier 90 is too fragile to make a good 3D trainer.

If you truly want a second 3D plane, you should be looking at a profile......and in particular a Mojo 60. That 75AX in a Mojo 60 would make a great second 3D plane. Later on as your skills improved you could simply replace the 75AX with a stronger engine.