"The heat generated by the RTG might create a pool/puddle of liquid whatever where there had been none before and introduce an undesired variable in the local environment."
"impact the planet on a subsequent go-round."
And this will NOT introduce an undesired variable in the local environment?
I personally say, go to Saturn - close and personal.
Not sure if you understood what I said. Cassini impacting Saturn - 'the planet' - will certainly have a far lesser chance of creating a pool of liquid stuff where there was none before than would an impact on an icy moon.