The octane will vary a lot depending on the alky mixture. Generally there is not quite enough octane to run a full 108 race chip, unless you have a well designed system.
Propane, when mixed with pump gas, comes out about the same octane level, ~ 100, give or take a few points.
Is the 5.0 supercharged or turbocharged? Propane is a little easier to tune and does not cause engine bog as easily, so I would go in that direction, except that it is more expensive initially. As time goes on the propane system gets cheaper and cheaper since propane is cheaper than alcohol.