Rent vs Purchase Used (selling after trip): 4-6 month trip

I'd go with the very used (under 100k miles) 05-09 option.
I'd look for a popular brand, floor plan and options and strike the best possible deal with cash and sell after i'm done with it.
Or for the newer units you can pull a loan on it but if you're buying from a dealer, the markup will likely put you upside down on the loan the moment you drive off the lot. If you know what you're looking at, try to purchase from a private party with cash, if you're lucky you may even come up or sell for the price you purchased it.