We have been at both times of year, though DW is a teacher and her spring break is in March so generally it is before Easter. We actually just got back from a trip this March and while it was busy and the parks were at capacity, it is the new, undisclosed but not 100% capacity so it actually didn't seem bad at all to us.
We are, for the first time ever, going twice in one year with the second trip coming in November, but we have been in December multiple times too. In the past I would 100% agree that December is busier than Spring Break but now with the reduced park capacity I am not sure that will be a factor any longer, and we are actually looking forward to seeing how different it might be.
Weather is sort of a subjective point in my opinion. DW and I like it cooler (we live in Texas, we get plenty of heat) so we don't mind the lower temperatures in December and have even been known to brave the quiet pool at night to float and soak away the stress on the feet from a full day in the park. I also feel that in the multiple trips we've made at each time of year we have had far more rain in March than December.
In spring we really enjoy the food booths for Flower and Garden and we have been known to make a day just wandering through the world showcase eating and drinking. And while it plays havoc with my allergies, all of the flowers, topiaries, and arrangements really are nice and make for nice photo backgrounds.
That said, there is something about seeing everything decorated for Christmas and, even though the cost has become crazy, the Christmas party is a lot of fun. We also like making an afternoon on arrival day (assuming we get in early enough and don't plan it as a half park day) of touring some of the resorts and seeing the gingerbread cottage at Grand Floridian or the carousel at, if I am not mistaken, Boardwalk. But close to Easter GF will have some amazing and elaborately decorated chocolate Easter eggs.
If you go into it with the knowledge that it is going to be busy no matter when you are there, and maintain the proper perspective, I think either time of year is great.
As for Genie +, the jury is still out on that. Last month was our first trip since that was introduced and we did get it on a few days. It is most useful, at least for us, at MK because DD is almost 3 and so she isn't tall enough for much of anything at HS, and a more limited selection at Epcot and AK. We did feel that it helped, particularly at MK and Epcot, but it is frustrating to navigate, not to mention being on your phone so much more.
