Both are fun - but BM66 is more fun to me due to the code and the many different ways you can play it.

BM66

Both are very fun. BM66 has very deep code, with a ton of risk and reward that keeps it fun and challenging.
The layout is a bit clunky, it is mostly just okay as far as the shots go. The code helps make up for it.

Deadpool has a great layout that is fun to shoot with very good rules. The code is reasonably deep, but not as deep and intricate as batman66.

Batman 66 offers a bit more variety of ways to play the game than Deadpool, but the Deadpool shots are more fun to shoot so its a bit of a toss up!

They both have pretty strong theme integration and great call outs so you can't go wrong with either there.

Deadpool is probably a better game to play multiplayer.

BM66 can be really long, so not super fun in multiplayer mode.

Normally I would say BM66 but you already have 2 fairly deep games that will be a long game to progress through most of it. As such, I would actually recommend Deadpool as it's not near as long of a playing game. To me having some quick players and longer journey games in a single collection is what keeps things interesting.

BM66 is diffidently deeper and probably the better code but (to me) doesn't really have any unique satisfying shots. DP is not near as deep but that dang lock shot just feels so good when you make it and the humor in the callouts are great.

In the end I only pick DP over BM66 to have something different in your collection but if you want to maintain the course you'll be happy with BM66 too.