I have status on both (Plat on AA, Summit on F9).
One big difference between F9 and AA is that, as EXP, you get unlimited upgrade to 1st class. There is no F on frontier, they are all coach.
Pluses on the F9:
* They have stretch seating, similar to E+ on UA. There is no stretch on AA. At 6'5", I need that
I believe it's a $25 upgrade fee, but it's free if you have status. Well worth the fee IMO.
* IFE is great. As a summit, I get complimentary TV, but I used to pay for it BS (before status). Used to be $5, but I think it's $6 now. No TV on AA (though you do have WiFi on some flights).
* More generous with carry-on than AA (bag dimensions for the sizer are larger)
* Planes are newer and cleaner (though I personally like the quiet ride of the MD80 in F).
So if it was me: $300 is a good chunk of change. I'd do F9 and pay the $25 each way to upgrade to stretch. As an added bonus, they usually let stretch seating pre-board, so you're guaranteed to have overhead space. Not sure if that's standard policy, but that's usually what I see when they start boarding is that stretch pre-boards with their elites. So that makes it $250 in savings instead of $300.
If you can't get stretch, then I'd stay with AA if you have good success upgrading to F out of ORD.