I got addicted to Ranch Rush and got bored quickly with Farm Frenzy. But it's really a matter of personal taste. I guess your best bet would be to view gameplay videos and see which excites you more.
I like them both. I find both rather clever and a good difficulty curve as you progress through the levels. If I had to pick one, I guess I would tip towards Farm Frenzy because it has profiles so different pepople can play. Ranch Rush only lets one person play it. The two games are quite different. Ranch Rush is somewhat like a traditional time management game in that you control a character who needs to perform various functions. Farm Frenzy is focused more on managing your farm (creating the right number of animals, processing food items, delivering items for sale). Both are time-based games. The quicker the better.
I love both games although I think farm frenzy tops it for me as I love the challenge of getting the gold medals and achievments. Has any one got all of the golds yet? There's one I just can't do and it's really bugging me. Must try it again. Please help