Anders,
Use both, that way you always have a backup. Regarding the noise with a lav, put a nice, thick foam wind screen on and tuck it under their shirt (just not under a jacket or scarf). The great thing with video as opposed to live sound is that you have much more head room to EQ and boost gain in the final mix. Every TV show now has the actors wearing RF for outdoor shots. Take a look at one of the 70 Law and Order spinoffs, you can always pick up the bump of the beltpack (yes, we saw it Mr. President) in the small of the actors back and sometimes even the mic element on their chests. You should experiment with mic placement for each shot, so bring a pair of high quality isolated headphones to listen to your takes. Dont worry about have perfect audio in the can, fix it in mix. Hope this helps.
-tim