For Marlins, specifically, I prefer a barrel shorter than 20".
It's mostly about balance and carry-ability; but being short and easily maneuverable is important, too.
While an empty Marlin 336 in .30-30 might balance fairly decently with a 20" barrel, things change once the magazine is full. They get muzzle-heavy.
The ideal compromise, in my opinion, is about 18". It balances quite well when empty, and balances very well when loaded.
But, if I'm cutting a barrel or looking for a short barrel, I want to go all the way to 16". (Or shorter, with an SBR stamp. ...But that's another subject.)
Balance is still great, and being a little shorter on the long end makes it that much easier to carry.
Capacity really isn't a problem, either. With a full-length magazine on a 16" barrel, capacity can still be 5 rounds (+1).
My still unfinished custom shorty 336 .30-30, with a 16.125" barrel and custom (extra short) 12-9/16" magazine tube, will still hold 4 rounds (+1).
My vote in the poll was for .30-30, as it's far more common. But...
.444 Marlin is another story. While I am not recoil shy, I don't exactly enjoy it, either. So, I do prefer to keep a little more weight on the front end with my 444s. Balance is muzzle-heavy, even when empty, but I like 18+ inch barrels. My own custom 444 (built from a 336 .30-30) has a 19" barrel.
My .44 Mag 1894 is one I haven't made up my mind about, yet. I really think that 14" would be just about ideal. But that's a big magazine capacity hit (-4 rounds), and I really don't mind it as it sits at 20".
But, don't forget: With any lever gun, the stocks on the rifle have an impact on balance.
A 336 with a 20" barrel and birch or polymer stocks is going to be more muzzle-heavy than the same rifle with walnut or laminate stocks.
That is also a factor with my aforementioned custom Marlins. The 444 was massively more muzzle heavy with a birch stock and plastic butt plate than it is now with its (oversized) mesquite stock and brass butt plate. And the shorty 336 .30-30 will balance better when finished with its maple stock than it did with the original walnut stock. (It is a very slim stock, and the maple isn't as dense as most examples.)
The biggest downside to short barrels: Shorter sight radius. (I see now that Drm50 also mentioned it.)
You win some. You lose some.
There is no free lunch ... unless you scope it.