the question here is an interesting one, and REALLY you have to look at your underlying intent. if you're trying to maximize your return on the $100 in the purely theoretical math world, you want to:
1. pick the machine with the highest theoretical payback.
2. play that machine at the lowest possible denominations that will give you that play table.
however, if you want to just have the highest theoretical possible return, you should throw that idea out the window and play for the biggest fricken return you can get. think: megabucks slots. (you may also get the added benefit of having your card points treated more kindly because you're not always playing VP.)
some casinos still use play-til-you-lose chips on table games (the golden gate downtown has these). the rule with those is you MUST use them on even money bets only ... and that's a big hint to you that you may well want to consider using casino money in cases where you are getting odds.
one way to look at it is to think that this is "free" money that they casino is offering you to win real money. if that's true, then you might want to go out and take a swing at the biggest possible money you can get. who cares if the return is zero? you know that there's a non-zero chance you'll be a multi-millionaire.
and, of course, there's mixes of this. play half for return, half for the grand slam.
i play for compromise of sorts ... when i get free play, i always play the highest possible denominations of the best possible return tables. so in a case like yours, i'd either feed $25 more into a machine and play a single spin of a $25 credit highest-returning VP machine ... or take four spins on the $5 machine.
don't be fooled by math types telling you there's a right and wrong answer here -- there's an entire psychological/thrill/POSSIBLE-return component that has nothing to do with theoretical return ... and my applied math degree won't tell you what that is ... but your gut will.
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