I've rented from both. Both are fine, IME.

When I last rented from Alamo a couple of years ago, I received a relatively new G6 for an intermediate. With Dollar, I can't remember what I booked, but I got a Chrysler 300 Limited. Alamo allows you to choose your own car on the lot, whereas Dollar will assign you a car (though you can probably go back inside and request a change).

I'd probably rate Alamo's operation a tad higher than Dollar's. For $20 savings per day, definitely go with Dollar. For $20 savings over the whole 29 days (less than a buck a day difference), I'd probably pick Alamo unless there were some overriding reason (i.e. I like the Dollar cars better or Dollar were offering triple miles or something like that).

BTW, they may be offering triple miles...I think Thrifty is, and they're sister companies. 150 miles per day * 29 days can add up. Poke around dollar.com to see if there's mention of triple miles and how to get them (you usually need to enter a promotion code during the booking process).