Deka vs. Interstate AGM for chassis?

Either is fine. I run a standard wet cell from Costco. Why AGM? (since you mentioned money)

AGMs make sense if you're in a climate where it gets regularly colder than 32F. I've used them for years for that reason, without a failure.

A group 65 starting battery is likely not going to be discharged much at all, and when they are not deep cycled their charge voltage is not nearly as important as if they were cyled to 50%.

I would personally not even bother with an AGM for starting if all it was ever going to do was start the engine and remain 99% charged for its lifespan.

Group 65 with their squatter less tall footprint, all seem to have higher CCA than an equal weight group 24. A group 27 or 31 footprint is smaller being not as wide, but they are a bit taller.

Sam's Club has some Duracell (East Penn) Group34 AGMs that aren't the worst option you could choose for a starting battery. Reasonably priced.

Check their charging specs wrt the alternator charging voltage. Some AGMs want 14.8ish and some want 14.4ish.

Interstate is a marketing company, not a battery manufacturer. I don't know off hand who makes Motorcraft AGM batteries. Maybe East Penn, Exide, or Johnson Controls. Deka is an East Penn brand...

Price being equal, get the one with the higher cold cranking amps rating.