Can't really help out with the first question regarding AMEX - But the $20/day rate is pretty good for 3 weeks

As for your side question - depends on what kind of car you like driving For me I always do a minimum ICAR (Corolla example car) reservation so that I can use GC (or be "upgraded" to an FCAR if they have spares - and majority of the time, airport locations do)... If you are content with a Ford Focus or the other B cars on the list, then go for it and save a few bucks.