There doesn't appear to be any large differences between the two. That said, prior to the 5, I had the 4 with cellular as well as wifi capabilities. The cellular rarely worked, so when I purchased the 5 after an unfortunate kid accident breaking the screen of the 4, I went with the wifi only to save the additional $10/ mo through Verizon as well as the additional upfront cost. Additionally, it does great in water. I swim with mine all the time. I've been very happy with it.
If you want to track your steps like you would in a fit bit, get the free Pedometer+ app and add it as a "complication" to your watch so that you can see your steps in real-time. Other complications I like include the weather, UV index, stock market, sunrise/ sunset time (I run in the evenings, so this is helpful to know how much time I have left), heart rate monitor and pandora. In all honesty, the feature that I use the most is quite embarrassing. You can swipe up and click on button and it will ring your iPhone, which I am constantly misplacing in the house.
Another thing that I like, but don't often use is the fact that you can pre-load Apple Music playlists/ songs onto the watch, connect wireless bluetooth headphones to the watch and listen to music played from the watch without the watch needing cellular, wifi or a bluetooth connection to your phone. So, if you were to go running, etc for example, you could leave your phone at home. If you decide to go with the cellular version and Apple has managed to really improve its cellular capabilities, you could leave your phone at home and answer calls/ texts/ emails through it as well with or without headphones as the watch has a speaker... but talking into your watch is a bit awkward. (If you would like to do this without the cellular option, as long as your watch is connected to wifi OR close enough to remain bluetooth paired to your iPhone, you can still do this). I haven't seen much in terms of reviews about the cellular capability improving though. The cellular add on reduces battery life in between charges, which can be a nuisance.
The only things that I would like to see in future models would be an increased battery life (it needs to be charged daily, which I usually do overnight... but it can charge in about an hour if I forget to remove it) and the ability to track change in altitude when running. It records "floors of stairs," but that simply isn't the same thing. This is a very specific running thing, but I like to track my mileage and pace with my watch. My runs typically consist of between 1.5 - 3 miles of very steep trails that go straight up a mountain. Without the change in altitude, it makes deciphering my pace difficult. This is coupled with the fact that I lose service (not Apple related whatsoever), so I can't really accurately map my runs either aside from just knowing what my usual routes are.
I also wish that they would offer child sized Apple Watch bands. I use the size Small band that they provide and typically wear it on the smallest hole. When it gets stretched out, I end up needing to purchase another one because the watch won't read my heart rate that well. I've tried some of the knock off bands from Amazon, but they ultimately fail within a couple weeks. Considering you are probably a dude from your profile, I highly doubt this would be an issue for you.