IMO, as your seatmate, I'd rather sit next to the child and enjoy the extra leg room (at the expense of having to... omg... interact with a child ) than sit next to the parent. The CARES harness should prevent him from poking/climbing on your seatmate, if that's what you're most concerned about.

My kid won't let me get away with it though. He loves to sit next to the window At 3.5yo, I wouldn't worry about putting him between me and a stranger. He's done this on the bus and in other similar situations and has never bothered anyone who didn't want to be bothered.