all games are not created equal

Lets get one thing clear here.  I prefer Double Dragon 2 over any TMNT game.  However, I have to admit that TMNT2 is overall a better game.  It''s a personal preference for DD2's dark gritty urban back-alleys rather than TMNT's bright colorful setting and characters. 
TMNT has a larger variety of characters, what with the different brands of Foot soldiers, mousers, robo-droids and what have you.  DD2 consists of brawlers, thugs, street fighters and the occasional ninja and wrestler. 
There's really not much to say between these two.  DD2 does tend to mix it up with some platforming elements, whereas TMNT tries to add variety with surfboarding (which doesn't actually changed the gameplay hardly at all).
TMNT wins based on variety, but I stick to my guns with DD2.