Welcome aboard, cmzimme!

I am a regular user of all three programs that you mentioned (Marriott, SWA and Hertz). My experience is that on any transaction with these you can only choose one reward program at a time. (Hilton has their "Double Dip" thing.) I usually go with SWA for my Hertz rentals since there have been lots of double and triple bonuses (1.0, 1.5 or as many as 2.0 RR credits per rental) lately, even on short rentals. You should check the Hertz forum for a discussion on using that reward program (search "ZE1".) When staying at a Marriott I always go for the RR credits as well (but then I have over 250,000 Mariott points right now). As you probably know, you're only going to get RR credits from SWA.

As an aside, you might have better luck posting this question on a more specific board (say Marriott). I suspect that one of our helpful moderators will do that for you soon.