It's a tough call. Have you checked to see what the blue book is on both bikes? They both have high miles. Do the sellers have maintenance receipts? Have you thought about saving a little more money and getting something newer, or is the 750 really what you want?
Well since they are both asking for more than blue book, and they both have high miles I would contact both sellers and haggle. Go with the one you can get a better deal on. The one with the bad shock is gonna cost extra money to get it fixed so maybe you can talk him down more. Have you looked into what the price of a replacement shock would be?
Bike looks good! It's not just one specific gas company, it's any gas that has ethanol in it ( wreaks havoc on plastic tank monsters). And as far as I can tell, there aren't any local (Fresno) gas stations that are ethanol free. Unless you get 115 octane, $6 per gallon gas.