We kind of addressed this in this week's podcast, since our core subject was Tower Defense Games and we talked about both Tap Defense and Fieldrunners as well as a number of the other major TD games. By and large though, it comes down to personal preference- if you don't want to spend any money Tap Defense is great, and it would also be the one to go with if you prefer fixed-path TD to mazing TD (though if you really like fixed-path and don't mind spending a few bucks, I'd recommend 7 Cities, especially since it's on sale at the moment...). Fieldrunners is more polished I'd say, but being a maze-building TD game it's going to appeal to some people more than others- it's definitely worth the money though.