I own (Premiums models of each) and love both games so you can't go wrong in my book with either one.
Batman 66 has a lot of nostalgic appeal for me and its audio/video features and artwork are fantastic! Its shots aren't always the smoothest, but it's very cool how many ways there are to try and blow it up and the game perfectly captures the campy fun theme. Gameplay will last longer and it's not as much "in your face" as SW.
As I've never been a Star Wars nut, I've been surprised at how much I like my SW as well. The shots feel SO satisfying to hit and the game has pure Ritchie flow! Perhaps it's because I never had high expectations to begin with as far as the theme, that I really enjoy the integration of the first 3 films into the gameplay, the light shows for each mode, audio, hyperloop, and the overall code - especially the risk/reward of going for high multiplier shots! Really enjoyable, but as pointed out, can be very brutal too.
I'm calling this choice a toss-up and therefore not casting a vote in your poll. As Eskaybee stated, Batman 66 will probably provide more variety to your lineup gameplay-wise, but you should really base your final decision on your personal experiences from playing both games.
FWIW: I started a similar thread over a year ago when I was originally considering these games for purchase. Although slightly dated now, you might gain some info from it as well.