I purchased the Wi-Fi + LTE model of the iPad Air 2 because I planned to use it outside my home and would likely need to be connected for some of the work I planned to do. I've been quite happy with it thus far.
LTE since the iPad 2. I need _MOBILE_ data, not data that's tied to a free wifi location. I also use my iPad for work, which means greater security than open Google wifi at a Starbucks. The real GPS chip is sometimes a plus. It costs me $10/month per iPad on my VZW plan with shared data.
Wi-Fi. For as little as I use it where there is no wi-fi, I just can't justify the extra cost of the device or the monthly charge to add it to my data plan. I also have an iPhone though, so if I really needed to get online for something I just use my phone. I think I've used the personal hotspot feature once.
LTE. You don't have to activate it, or you can activate it as needed and then shut it down the following month. And it adds true GPS and likely increases resale value. It also offers greater user flexibility.
I would also vote LTE. I bought a wifi Air 2 and never used it since I use my rMBP at the house. The odds of it being useful increase when it can be used outside the house. Yes, I know, you can use your iPhone as a hotspot.
Last year I bought my 64gb Mini Wifi, I wished almost immediately that I had gone for the LTE version instead, I don't really use it outside of the house because of that limitation and when I am going to be somewhere with Wifi I just bring my rMBP. I thought about replacing it but considering that I paid $800 for it and now its worth about $300 I don't think that is really an option,
My opinion, get the LTE version and even if you don't think you will ever use it without wifi, its always good to have the option