Definitely TMNT for me. Yes, it is fast and a can be mean, but it has so much flow and is so very satisfying to shoot. The code is amazing, lots of diverse modes broken up with many side goals, plus a ton of different game types to try out (co-op, wizard mode challenge, a speed run challenge, etc). So many ways to play. I really love the game, and I might currently call it my favorite of all time. Definitely top 5 at worst. There are only a few things I would change. The animations could be better. I do not dislike them, but most of them aren't amazing. April stands out as the worst, but on the flip side there are some that are really nicely done. I would say this is probably the weakest aspect of the game though. I'm also not in love with pizza eating contest button mash mode. It's ok, and I like the risk/reward of taking your hands off the flippers with the ball in play, but...ehh. I wasn't in love with it on SW and I'm less in love with it here. Those are minor criticisms though in the scheme of things, and do not hurt my enjoyment of the game at all. It can be a punishing game when you are off, but when you are on...man I can't think of a game that feels better. Bust out a perfect 1-2-3 Foot Combo and tell me that doesn't feel amazing. Makes me smile just thinking about it!
JP is a good game, and obviously one of the recent "golden boys" around here. It is a more unique shooter with a very deep and unique ruleset, but I do not find it nearly as satisfying of a game, both layout and code-wise. The paddocks, which are the main focus of the game, get repetitive and are not all that exciting. However, the other aspects, such as T-Rex modes, Control Room modes, multiballs, etc, are much better. T-Rex modes especially are awesome. If the paddocks were half as fun as the T-Rex modes, it would be a heck of a game. Unfortunately they are pretty flat by comparison. I like that you can choose your path through the paddocks, and earn different rewards. It's nice that the dinosaurs act differently, and the difficulty ramps up in a clever way by having to set more traps, calling in the helicopter, etc. But in the end, even with those differences they just sort of feel the same after awhile. The shots are diverse and unique, but not the best "feeling", especially compared to a game like TMNT. It's also a very hard shooter, and if you aren't hitting the shots cleanly, it can feel super clunky. As for the art and A/V assets...I like them quite a bit. I actually like that it isn't movie based, but rather an extension of that world. Going back to the "boring" paddocks, I find the default "truck view" screen to not be great, but I really love some of the other animations. The ankylosaurus batting the ball back into play for a ball save is a favorite, and others similar are all great. Very well done by the animators.
JP is a tough one for me as I had a super strong honeymoon period with mine, but fell out of love quick and sold it after about 6 months. Yes, I got through the bulk of the game (never got to Nublar in-game, but had done everything needed to get there multiple times, just not all together), so it wasn't a matter of needing to play better to see the cool stuff. However, I have found myself missing it some lately and wondering if I gave up on it too soon. I wouldn't be surprised if I ended up with another one someday, but I would never give up TMNT for it. Honestly, just talking about it here feels like I'm talking myself into giving it a second chance, LOL!
Sorry for the long ramble. In the end, both are great games. JP gets more love in general, but I am definitely on team TMNT. As always, buy what makes you happier! If truly torn between them, just go with whichever one is easier to get your hands on now, and add the other when budget allows. Both will be pretty big runners I think, but with the popularity of JP, I am guessing they will be a little easier to get into the future. If stuck on NIB only, I might lean towards TMNT now. We know there will be another JP run in spring/summer. Not positive there will be another TMNT run or not. And if you buy one and feel it was the wrong choice, both should be easy to sell or trade for the other. That is one of the best things about this hobby. You are rarely stuck with what you buy and in most cases do not lose a ton money, even if buying new. Good luck!