Again, it takes a long time to learn all the ins and outs of loylaty programs and the decisons are made individually. Delta might be good for you , but if someone else always flies AA, getting Delta skymiles doesn't really help them. fwiw, I have perhaps done the combo hotel/car twice out of hundreds of bookings. YMMV - your miles may vary. Only you can make that decision. That said, be aware that if you have a rental car from certain companies(even if you did make your res trhough HHonors), you can get 250 points per stay by showing the rental car keys at the registration desk for HHonors stays.
http://hhonors1.hilton.com/en_US/hh/.../carrentals.do