I've got both a 1895 Guide Gun, and a Rossi M92, so I suppose I'll weigh in. They are both good, but for different reasons.
Rossi
Pros:
1)Extremely light (I have revolvers heavier than this thing)
2)Short lever stroke
3)More rounds in the tube
4)Cheaper factory ammo
5)Costs less
6)Stock has good LOP
7)Decent factory trigger break (imo)
8)Better balance
Cons:
1)Dove tail on the barrel is oversize (most aftermarket sights will not seat snugly)
2)Nightmare to take down and clean, complex dissassembly, lost of small parts.
3)Cannot mount sight base, or aftermarket sight on top of the receiver (though bolt and tang mounted sights exist for it)
4)No factory previsions for a sling
5)Wood the stock and forend are made from are of inferior quality
6)Brass goes flying everwhere upon ejection (fun to watch but if you're going to reload that brass it's annoying)
7)Safety is miniscule poorly made (can even unlock itself if the hammer falls on an empty chamber). It's far better to rely on an empty chamber or the half-cock.
Marlin
Pros:
1) Dissassembly involves taking out just one screw
2)Stronger round
3)Great quality stock
4)Stronger action
5)Smoother action
6)More maneuverable in the brush (shorter overall length)
7)Sling studs
8)Crossbolt safety (Works like any old crossbolt safety and is superior to the Rossi bolt-safety)
Cons:
Heavy
1)Newer ones tend to have defects (I had to have the front sight holes re-drilled on mine)
2)More expensive
3)Less rounds in the tube
4)Long stroke
5)Stock has a sort LOP
6)Trigger pull is heavier than it needs to be
7)Trigger can flop forward and backward (there is a part to fix this)
I like them both a lot. If you have bear in your area and want to take a companion pistol of the same caliber, your options are limited for the 45-70. Although there is always the BFR.
Factory .44 is cheaper than factory .45 iirc. Definitely a lot cheaper than factory 45-70. I usually get HSM 430 grain for bear loads in 45-70, that's 42$ for a box of 20. Leverlution is probably the best deal at 30$ for a box of 325 grain rounds that are quite accurate.
For .44 I get the cheap blazer stuff mostly. I want to say they're 240 grain, something like that. 35$ for a box of 50, but they're steel case so you can't reload them. For bear rounds I go with 305 grain hard casts also from HSM, which are 47$ for a box of 50.
Another thing to consider is the recoil. Since you'll be loading that won't be such a big deal, but 45-70 can be either very pleasant to shoot (Leverlution is), or very unpleasant. The .44 on the other hand is always pleasant. The overpressure and muzzle flash are worse on the 45-70 as well. It's just a louder, more powerfull round.
Of the two I'm tempted to say the Marlin comes with a better stock sight set, although being collapsible I've found it prone to collapsing when firing rounds which exhibit heavy recoil, this problem was solved when I replaced the stock stuff with Skinner Sights (can't say enough good things about these).
The Rossi buckhorn covers up the a good deal of it's sight pictures, which is obtrusive. The front sight on the Marlin also has a nice sight hood.
Another interesting thing to note is that I think .45 colt can be loaded for more muzzle energy than .44, I've heard that at least, although I don't have any experience with that caber. I'm partial to .44 myself.
It's a decent round, but compared to the 45-70 it doesn't look so beefy.
The .357 Magnum on the far right looks like a .22 by comparison.
At the end of the day I happen to think the Rossi is more fun at the range, but I'd rather have the 45-70 with me when I'm out in the back country, fishing, hunting, and so forth.