Well I own both IM and TNA, so I'll tell you my experience. These two games get played the most in my collection over the last year, along with FG from time to time. What do they all have in common? Short game times and a ridiculous amount of getting my ass kicked. That's the kind of gameplay I love.
Most pins have some degree of rinse & repeat in their rules, but TNA thrives upon it. Start reactor, critical reactor, destroy reactor...with various score maxing strategies & multiballs mixed in along the way. It's just how the lighting & music is integrated into TNA that makes it so appealing (along with mutiplayer), and the fact that it is easy to learn, hard to master. That gets compounded when you start a multiball when you didn't mean to, which I find pretty frustrating.
IM is straightforward with it's rules as well, but it is just SO much more appealing to me. The modes are easy for a beginner to grasp, but are deeply layered for an experienced player, because they are so hard to complete. Take the Whiplash multiball mode for example. Sure it's easy enough to start, but how many people are actually able to complete all 18 shots for the Superjackpot? I know I never have. It's like this for the other multiball modes as well.
And the wizard modes are my favorite in pinball. It's not impossible to get to Jericho, but it's challenging, and it sure is fun to rack up millions with each successful shot. The fact Jericho is a single ball mode is brilliant, because the points are right there for the taking with skillful play. And scoring the timed 35M Do-Or-Die Hurry-up on a single ball is one of the greatest satisfying thrills in pinball. IM is a perfect example of how good coding ideas can elevate a game to greatness.
Plus IM fights you back the entire time. The War Machine kickback is brutal. The game is fast & flowly even with two magnets. Want an extra ball? Good luck earning one. Want to get to Do-Or-Die Multiball? Maybe when pigs fly. This pin makes you earn EVERY FRICKING POINT. An uniformed player might look at IM and see a "stripped down" modern pin & never give it a chance. I see pure brilliance from Borg design & Lyman coding. It's a modern classic.
Can you tell which way I'm leaning here? IM all day! I'd never sell mine.