I might be able to offer some perspective. I have two pins in my collection, one of which is a Cyclone that I will never sell due to sentimental reasons (much like your Taxi). I've tried other system 11's next to it but they seem the same. Sure they are different games, but having two games from the same era doesn't provide enough variety for me.
I'd suggest getting a dmd game as its much deeper and will do better in a small collection. Price is a big obstacle so finding something you like will be challenging. Maybe you can find a World Cup Soccer for $2500 - $3000? Personally, I'd avoid xfiles like the plague, but some people love it.