If you are planning on letting her paddle around the pool without you holding on, then I wouldn't consider anything besides one of the life jackets that has a strap between the legs.
I used to be a lifegaurd and they warned us pretty severely about the dangers of the arm bands and swim rings--the kids can slip through them so easily and they give parents a false sense of security--it only takes a moment for your child to drown (Not trying to scare you, but it scares me when I see parents pop their non-swimmer into arm bands and then turn their back and chat with friends while their toddler swims around in the pool).
I haven't even ever used a lifejacket with dd (except when on FIL's boat) because I'm worried that she will get used to floating when she goes into the water and forget sometime when she doesn't have it on and just try to jump in to water sometime on her own. When we swim, I am within arms distance of her at all times--I just don't want to get distracted and forget that I'm responsible for her life right then.
~Kim