Hey Kim
I've actually owned quite a few fitbits over the years. I've had two fitbit one's, one Zip, one Blaze and now I'm using the flex 2.
The fitbit one was great for tracking sleep and it also did stairs as well as steps which was cool. I did find it kept coming off unless I clipped it to my bra. I lost my first fitbit one and the second just ended up going through the wash one to many times and stopped charging properly after that (they aren't water proof).
The Zip was my third fitbit and I loved it, all I was interested in really was steps for a pedometer challenge at work and for that it was great and you don't have to charge it, it has a watch battery and you change it every 6 months I think, so you don't have to worry about it dying and its the least expensive and comes in funky colours which makes it easy to spot in the morning when I'm not wearing my contacts. I unfortunately lost this one as well, I accidentally dropped it and I think it fell down in a floor vent.
The fourth fitbit was the blaze, I actually debated over getting the blaze or the alta for month before I made the decision. In the end I purchased the Blaze because my boyfriend was getting the blaze and I wanted us to collect and compare the same data. I loved the blaze for him more than myself, it fit well looked good and is great that it collects the heart rate data. For me I found the blaze large and uncomfortable to wear when I slept because I sometimes lay on my hand. I did however love it for day time use, all your information was right there at the tip of your fingers and it does auto recognize some forms of exercise, you also can get texts and it vibrates for calls. Its unfortunately not waterproof though, I think water resistant. The blaze I took off moments before walking down the aisle for my friends wedding and I put it in a bag an never saw it again (Bummer).
Which brings me to the flex 2, which I think may be the best fitbit for me. I purchased it for three reasons, its waterproof so I can wear it anywhere (making it harder to lose) and the band is smaller than all the other fitbits which has made it way more comfortable and the price was reasonable. It tracks steps and sleep but not floors/stairs. It also auto recognizes most exercises. It came with two bands (one large and one small) so the large one is actually big enough that I have been putting the flex on my ankle when I use my stationary bike (otherwise that exercise it not recognized). Oh and if I have to go out somewhere and dress up there is always the option of buying the necklace pendant that holds it if I decide to go that way. The one thing the flex 2 doesn't do is show you any info on the device itself, you have to look at the app for details (which isn't a problem because my phone is always nearby). It does however vibrate for move reminders, texts, calls and when you hit your daily step goal.
I've read about the charge and if you are super into fitness and think you really need all the extra information than by all means go ahead and get that one, just be sure you are someone who can handle sleeping with something bulky on your wrist (maybe test it with a bracelet or something) and second keep in mind the fitbits with the gps tracking have to be charged more frequently. Fitbit has made a fortune from me lol
For all the money I have blown in lost fitbits though I just want to say every time I have one I lose weight and every time I lose one and don't replace it right away, I gain weight, so for me its been worth it.
Good luck and I hope you find something you like and I hope this helps. Its not an easy choice when you are picking something that's going to be attached to you every day all day.
Serena