Neither of them are any good, way too slow paced and doesn't offer a real challenge. Fieldrunners is even more ridiculous in the fact that you don't get the option to fast forward until the end of the game. Geodefense is better than both of them.
Haven't played 7 cities. I really enjoyed Fieldrunners a lot, but I got it right after it came out (might have been my first iphone game). Since then a lot of much better games have come out for tower defense and all genre's really. I haven't played it in a long time now. A year ago it was one of the best on the appstore imo.
you busted a funny I also think geoDefense is better than both of them. I believe 7 cities has fast forward buttons but i just dont like it too much. Even then, it does feel slow. Fieldrunners gets old fast. The Creeps! is also better i think
Fieldrunners was my favorite game of all time when I first got it..I was new to TD back then and thought this game was amazing. After all the other TD games came out this doesn't seem anywhere near as good (the creeps, geoDefense). between the two though I'd pick 7 cities.