Ages is a blatantly longer game in practice, but also has an easier to understand gimmick. It's a single back and forth thing.
Seasons, while shorter, has some really cool items too(as do both), but a much harder final boss(unless you get a bit lucky). Using the Rod of Seasons means you have to cycle through up to 4 different overworld parts at times, and sometimes a puzzle requires you to do this among the same area, not just once in a while.
I'd say the worst factor of Ages is how odd the controls are for the Mermaid Suit. It goes from direction and A to nothing but directional inputs. This is a massive change from earlier. That said, this also can be easier for some people compared to the Flippers. Generally, one of them won't be easy to use.
Seasons doesn't have any truly awkward items, though the Magical Boomerang can sometimes be a little wonky at best(it controls like the Mermaid Suit, using the D-Pad).
Overall, they're both very fun. Great music, great story, neat characters, and lots of silly rings to collect. Biggest downside is that if you want 100% completion, you can't use the released versions on 3DS or the Switch, as they specifically are coded to follow Game Boy Color rules. You need to get physical copies to play it on the GBA itself. That said, the unique stuff are two "nothing" rings and some Gasha Seeds. Thankfully you can use a password for a linked game to get them, but it's still pretty annoying.
