Santa Maria Novella - Climb the dome or bell tower?

Do you mean the Florence Duomo? Santa Maria Novella is not that tall, and it's nor really domed. Assuming you mean the Duomo, I think the dome is slightly preferable to the campanile. On top of the dome it slopes down dramatically before you and you get to see the inside of the cathedral from up high. The campanile is a sheer drop if you look out. You can't really see the structure from within the structure. You should think about doing both though. And for sure do not miss the baptistry.

If I had to choose, I'd say the dome, for the reasons that vftravels mentions. But I'd consider doing both, if possible -- My wife and I (late 40s) did both last time we were in Florence on consecutive days.

Try the dome your first day. Decide on whether to do the bell tower on the second day. There are two or three nice 'rest areas' on the way up, so you might find the climb a little easier.

Right: you probably mean Santa Maria del Fiore in Florence, which is commonly referred to as the duomo. "Better" is a subjective term but you do need to have advance reservations for the dome. Information on the complex (duomo/dome, museum, belltower, baptistry, crypt) is here:

https://www.museumflorence.com

And I'll agree on the baptistry: it's a must see!!!
Personally? I don't personally see a big need to do both but most folks lean towards doing the dome.

I prefer to climb the Campanile (Giotto's bell tower), as you can see the dome from there!
Also, not so claustrophobic, and lots of places to rest on the way up.
I've never climbed the dome, as I don't like small narrow spaces.
Santa Maria Novella has nowhere to climb to, as far as I know; so you mean the Duomo(Santa Maria del Fiore.)

I have done both. I found the dome to be physically easier than the bell tower. The bell tower has steeper incline on the stairs. It's a cardiac challenge for those of us middle-aged and older.
Another overlooked view is the tower at Palazzo Vecchio. This is good for folks with the Florence card. Otherwise, there is an admission fee to Palazzo Vecchio which is a good historic site.

We did both - Dome in morning, bell tower late afternoon - both equally as beautiful. bell tower a little more claustrophobic but still worth the hike!!