I went with the F nib. I have briefly tried an EF nib (on a Safari), and didn't mind it, but I worry that it might feel a bit dry and scratchy for regular or prolonged writing (for my tastes). I also find that M nibs are a little bit on the wet side (depending on the ink), so F seems like a good middle ground.
Any ink recommendations?
Nice. I have 3 swipes now, and I absolutely love them. They write nicely, hold a great amount of ink in their oversize converters, and look great. The only thing I don't love is the clip, but I'm getting used to that too.