The clown pleco shouldn't care which tank he's in as long as there is also some wood for him to chew on. The plants just need similar lighting par to what you have on the 29 as long as you continue with whatever fertilizing you do.
There is always the choice of adding another Hypostomus Plecostomus ( the big ones) and letting it grow in the 75.
As for the Guppies, if you do not have a store that will take them from you, you always have the option of putting a fish that eats them in the tank.
But the reality is this: you need to choose which tank will give you the most pleasure to have running. The bigger tank will require larger water changes which will take a bit more time than in the 29. The 29 is still a relatively small tank so it's always possible that you will get or want a fish specie that requires a larger tank ( which you already have) and so you would need to get another larger tank. So there is no easy answer to which one to take down.