This would be a no brainier if you owned a larger collection. TNA is more unique and would easily be my choice. In a small collection with only EMs it's really a toss up for me. I like both games. I like the idea of buying an AC/DC now and getting on the list for TNA. You could then sell your AC/DC and pay for TNA. If you decide you want to switch back, I think you'd have a pretty good chance of recouping your cost on TNA. You could likely sell it to someone further down the list at that time.

I still feel fairly new to pinball and if there is one lesson I've learned in that time, it's that the depth of rules has little to do with how long term a game will stay in my collection.

No I think that's a fair reason to not enjoy TNA. The split playfield design makes it play super fast, but if that's not what your into then there are certainly other games. To make it even harder, you're often playing those during multiball.