Hey aimcat we are the same age group as you and we have been on different lines and they all have their pluses and minuses.
We are going on the msc seaside soon (can't wait). We have done carnival horizon and msc armomia and both were good.
But big differences for us was the food. Buffet food for breakfast and lunch was actually alot nicer on msc than carnival but as we are Brits thats a very subjective point. However horizon Has alternative free lunch venues.
Dinner in the main dinning room we found better on the horizon and the service was alot better too. However things like a new york strip aren't free on the horizon but they are on the seaside. The breakfast menu in the main dinning room seems more extensive than on the horizon.
You will also find that people go out alot more late at night on an msc ship (it's a European thing) than compared to what we noticed on the horizon.
We loved the free room service and the certain free items that I could order on the horizon. I think they are stopping free room service now? MSC still does free room.service for fantastica experiences and above and I have seen online there is alot more free food included than we got on the horizon whether it's better room service food I have no idea.
Only one ice cream machine on seaside, there were loads on horizon and they had freshly made ice cream in the buffet - 2 different flavours every day.
The cakes in the buffet were large slices from a cake - loved the German chocolate cake. MSC like most lines has those fancy little cakes.
The sports bar on horizon was great and so was our bar tender. I watched a couple of football matches including the champions League on there. The sports bar on seaside looks even better and has free chips there. The horizon didn't have any free bar food and if you didn't get there early for the match it could be hard to see one of the numerous TVs......the seaside seems to have more space and more TVs plus it has more private booths with little TVs in them so technically it should be better if they have the football there.
The horizon has like one slide I think the seaside has alot more.
Muster drill is alot quicker on MSC. Horizons drill was in a smoke filled casino and as a non smoker with kids that was very annoying as we had no choice to avoid it.
Lifts on Horizon were not only slow (most ships are like that) but there were annoying to operate as you couldn't just press a button, you had to go to a touch screen and tell it what floor you wanted and then it was meant to tell you which lift to take. You would think this would make things easier but the system never worked properly.
If you like live music the music was good in the horizon and so too were the comedians. MSC live music I don't think is as good but the night life is more livier on MSC late at night as Europeans like to go out late. MSC don't have comedians as such but they do have live improv and from the YouTube videos I have seen it looks pretty funny.
Design wise the seaside looks alot nicer than the horizon. This point doesn't bother me much, however a running point I found before cruising was how much horizon hyped up their ship compared to seaside which meant we didn't enjoy it as much as we should of done if that makes sense?
Last but least - the beds. MSC beds are very comfortable compared to what we experienced on horizon, that's not to say they were uncomfortable just not as much to our liking. I prefer a firmer mattress but if you don't then you won't like the beds on seaside.
If I had to decide which ship to choose I would base it on what is important to me. The price? The food? The entertainment? The ports? Or the amenities and if you still can't decide go with the cheapest!