It is relaly up to youa dn how confident you feel about letting her free in the water.
At her age I'd suggest nothing as you want her to be in the water not floating above it. Once she is a little older and moving legs etc the ring and arm band combo is great (and cheapest option) - can't fall through as the arm bands make her too big to go through the ring.
The floats with cut out for legs are rubbish as they just float on the water and only their legs from the thigh down are in.
I bought the life jacket for my son and never used it as again he was too high up in the water.
I went to a baby & toddler session and they are great for making you confident in taking your baby into the water - it usually your fear not the babies that make us use loads of swimming aids.
Hope it helps but it is up to you.
good luck and hope you both enjoy it
B xx