I am gonna make a list of what is better about the two games:
New Leaf:
* More upgrades and buildings to unlock.
* The overall dialog and spontaneous letters were better and more heartwarming (there is some good dialog in NH, though, do not get me wrong)
* Villagers had better initial shirts
* Lots of amazing PWPs to unlock (though poorly executed)
* Lots of amazing furniture series that got removed in New Horizons
* Writing letters actually improved friendship (I read that is not the case for NH)
* Villagers asked for favors more often (unpopular opinion, but I actually like this about NL)
* Easier to get villager pictures (in my opinion, anyway)
* Exterior for your house has more options
* House is more spacious
* Easier to make bells
* Gems
* No tedious Nook Miles to worry about (it can be tedious to get them in NH)
* Major holidays are executed better, holidays in NH relied a lot on crafting.
* The bug-off and fishing tourney are executed better
* Villagers will smile when you sit next to them, if their friendship is high enough
* Similar, the megaphone is a thing here. Villagers will wave when you call them out, if their friendship is high enough
* Just overall had more features like working part-time at the cafe
* Amiiboing villagers is instant, whereas it takes three times in NH
* Tools do not break (other than the regular and silver axes)
* TORTIMER'S ISLAND!!!
New Horizons:
* More creative freedom - You can terraform, move houses/buildings/trees, relocate, place furniture outside, break rocks, etc.
* The graphics, detail, etc. are mindblowing
* New bugs, fish, flowers, deep sea creatures, etc. (though some got removed)
* You can give gifts to villagers daily
* Apparently, you can comfort villagers when their sad, and calm them down if they are mad (I haven't done this yet, but I heard this was a thing from a friendship guide)
* The museum got a major overhaul, which it is absolutely stunning
* Diving is executed a lot better
* Although collecting Nook Miles can be tedious, it was still a nice implementation, as it encourages us to reach, well, milestones.
* No unnecessary inconveniences like the train passing by on Main Street when I want to get there, or tripping due to bad luck.
* Farming and Cooking
I always said that I prefer NH over NL, but that is probably due to the fact that NH is newer. Truthfully, it is hard to choose. They both have their own charm that makes them different in a good way. If Nintendo took what is better about each of these games along with some other features we as a community have been wanting, it would make for a very kick-ass Animal Crossing game.