Personally I'd like to use them both along Crono but I think not many people do.

Both Marle and Frog have Water elemental attacks and healing magic that are quite useful.

Which of these two do you usually prefer? Marle is more supportive and Frog is more offensive.

Frog also has Heal which mediumly heals entire party and also has Cure2 which heals one's HP almost upto max. I used him and Marle all the time they can have some good curing combination skills together.

I agree about preferring attackers than mages. In Chrono Trigger really key part of the fighting is to coordinate timing of three different characters so that they can use combination as soon as possible if wanted to, without wasting time of waiting till another's gauge fills up. If you are using fighter, you are likely to be not scrolling down your tech options all the time, so it is easier to have smoother fight.

I agree that Frog has high compatibility with Crono. Actually, Crono has high compatibility with Marle as well. That's the main reason why I use Crono, Frog and Marle in the game.

Crono and Frog combination yields powerful attack while Marle can heal. Crono can damage and Marle and Frog can heal in really severe situation. Crono and Marle can do some Ice Sword damage (they are pretty strong if u havent really noticed. It is really basic attack but its absolutely worth the MP points used) and Frog can do Heal if everyone needs moderate healing.