I also own both games and 7 Cities.
Regarding TD games with "real" graphics I must say, that I'm tired of waiting for the next Fieldrunners update, than the next and so on. They are just too lazy. I still have Fieldrunners sitting on one of my homescreens, but regarding to Tower Defense games, I only play 7 Cities. It is fun, graphics and sound are great, maps look great, controls are great, update possibilities are great etc. etc. etc. I LOVE THIS GAME!
Of course I also loved Fieldrunners, but I played the new map only for two hours and then lost interest again. FR is a great game, but it's a game for beginners. It has no depth. There's only a limited amount of mazes to build "right" and then you can basically stop playing this game...
I also bought Strategic Assault and Warfare Incorporated. SA sucks and WI is great, but I only played it once, then never again. It does not feel home on the iPhone like Fieldrunners or 7 Cities do. This might change with the graphic updates. Also it will get cool features like new missions and multiplayer and the game already has years of history and developement, so these people know what to do.
Conclusion: Buy 7 Cities and wait for the WI graphics update, then also buy this. 7 Cities is more pick-up-and-play, while WI needs longer sessions for each mission, so different moments when to play each one