I own both. I try to do 10 games per day as practice etc, and my thoughts are sort of based around that. First, you'll play each game of BM66 wayyyy longer. I can finish 10 games of TWD very quickly and feel like I made no progress at all haha. BM66 is more forgiving. With all of the different scoring strategies, I actually think BM66 does have/will have deeper code than TWD. Just more to do in the code. I think TWD is a fun game that kicks your butt if it's set up tough, and Batman is a tough game if you try to do anything to complete various code objectives just because it demands a lot from you shot-wise. It never feels unfair though. TWD feels unfair sometimes.
Also, and I say this while fully loving my TWD premium, the left ramp shot on TWD premium is absolute garbage and I've never been able to get mine to work perfectly. It's a major turnoff. There are ramp extensions and things like that I haven't been able to add yet because more are being made, but it's still worth considering.
Personally, after a bunch of play on both, if I had to buy again from scratch and could only do one, I would choose BM66. The screen is great, the callouts and music are fun, it can play long but that also isn't necessarily a bad thing. I do tend to get a little bored trying to cram 10 games in a row just because for BM66 10 games in a row is a lot, but that's my practice schedule fault, not the game's fault. It's a beautiful machine, fun to shoot, and my own experience is that the worst mechanically I've had to deal with is sometimes the ball gets stuck in the turntable but ball check always releases it. TWD is frustrating, punishing and has more unique shots than BM66. Just depends what you like when you play. I'd still go with BM66 though.