I was an Omron user before I jumped to Fitbit just over a year ago--Fitbit has been amazing for me (the web interface is really neat and seems to keep me on track).
I've had the legacy model (but I broke it), upgraded to the Ultra, and don't have plans to move to one of the later models. You only need one--believe the Zip has less functionality (like it doesn't track sleep), but it's far less spendy than an Ultra or One and smaller. I'm hearing the One is better than the Ultra in terms of water resistance (these aren't water proof, but sweat has never bothered one of mine).
Lots of folks clip to the bra, some clip to the waistband or put in a pocket, I wear mine on my underwear (less standard, but I've gotten into the habit hiding it under clothing, and I've found it's a lot less prone to falling off--for me anyhow). Because of the price, lots of folks swear by a pedometer "leash" as well.... just in case.
Hopefully this is helpful. Oh, and in breaking news, Fitbit links to Sparkpeople now--SP food can be uploaded to FB site (FB database has always been a little weak) and FB fitness/weight can be uploaded to SP). LOVE LOVE my Fitbit; can't say enough positive things about it (including the customer service--they have replaced more than one for me at no additional cost as well).
Good luck on making your decision,
| current weight: 145.3