Not sure which is more popular, but after some deliberation, I chose the non-cellular S3.
After using it for a year, I have no regrets going with the non-cellular, as I always have my phone with me when I'm out and about.
I suppose if I was still into running, and didn't want to use the armband thingy that's buried somewhere in a drawer, I'd prefer the cellular. If I felt that calls would be important to make or receive when I'm running.
My preference is to have the phone with me at all times, for making and taking calls, returning texts, and a bunch of other uses.
Several times I've answered the phone from my watch, when the phone was in a different room of the house, and it's a neat feature. More frequently I like being able to refuse calls from numbers I don't recognize without having to take the phone out of my pocket. Happens more often these days, it seems. And, since my phone is nearby or in my pocket, I don't need cellular to do those things.
I look at the Watch as an iPhone extension - not strictly a watch, and not a standalone device. My S3 GPS fills that niche for me swimmingly.
My suggestion to anyone pondering which version to buy is to do a realistic assessment of how you would use cellular in the Watch, then decide. With either choice, you still get what I consider to be one of the best bits of technology Apple has to offer - the Apple Watch.
Enjoy!