Would you rather have a 1968 Chrysler 300 or a 1965 Mustang?
The Mustang would be worth more than the 300 would ever be, if that is a concern of yours. If you bought the 300 you are going to stand out from the crowd unlike the Mustang plus you have the big 440 engine in the 300 that already has tons of power and easily modded to make more! My grandfather has a '68 300 with a TNT 440 that produces 375 horsepower and is very fast, but does so with class. I may be a little biased even though I do own a Mustang. It all boils down to this: Chrysler 300 = expensive/hard to find parts + lower retail + a truly unique/uncommon vehicle, Ford Mustang = fairly cheap/easy to find parts + higher retail + a very fun but fairly common vehicle. If I had a rare Mustang I would not get the 300, but that's just me. Follow your gut and you will make the right decision.
hope this helps.