I'm a frequent visitor and just did my first NYE this past year and I have to say that the crowds are beyond intense. They move along surprisingly well but it's just a sea of humanity and was a little stressful keeping track of multiple kids. We were a large group of 10 and there was just no way to stay remotely together when walking around the lagoon. There's a twitter account EpcotNYE (unofficial acct) that "comes alive" only for New Year's each year but you can check it out for some pictures and video of the crowds.
If your fiancé doesn't like crowds, it sounds like remote viewing of the fireworks might be a better plan. For remote viewing, I think the Poly beach is probably your best bet.
If you think you may want to try to go to a park, then I'd say Epcot is the better bet. MK did enter phased closure this past NYE (but not sure it ever closed to resort guests). We stayed at Boardwalk and found the ability to walk in/out of Epcot to be priceless. Early on in the day we were able to go in and do a few rides with fast pass. We went back into the park for dinner - we ate at La Hacienda and by happy accident ended up with a view of the first round of fireworks (an aside - I think an ADR is an absolute must - the lines for quick service were crazy and I have no idea where people found space to stop and eat) and honestly decided after that, that the crowds were too much for us. So we went back to the resort. There was a DJ and huge dance party for the kids (or young at heart) on the green and we bought a couple of bottles of champagne and some paper cups and just sat out on the big porch and chatted while the kids went back and forth to the dance party. We went down onto the Boardwalk for the final countdown - they were handing out noisemakers - and while we saw the top of the fireworks, it's not really an ideal view. It was enough for us but again if fireworks are the priority, I think I'd say stick with the Poly. I'm not positive they offer it on NYE but if you had an MK facing room you could even order in Ohana and just hang out!
This was long-winded and I'm not sure if even helpful. Happy to answer any questions about our Epcot experience last year. I'm sure whatever you choose to do will be magical. You just have to go in with the mind set that this is not a typical day in the parks. Oh and congratulations!!