Is old vs new really a factor?
I have both a Rossi 357 and a 1894 Marlin. They are both good rifles. My Rossi is a little slicker and maybe a wee bit trimmer but the Marlin is easier to mount various sights onto. If you get a good one I think you will be happy with either. Unfortunately, yes, when they were made is a factor in both rifles.
Rossi quality as been up and down over the years, for a long time they seemed to get better and better and then recently they apparently declined again... You can roughly determine vintage by looking at the wood and the sights. Much older examples tend to have darker stained wood and newer models have lighter wood. Newer Rossi's (roughly in the last 5 years) have semi-buckhorn sights vs plainer sights on older models.. Most Rossis are good rifles but there are a few lemons out there...
With 1894 Marlins the older (and usually better) rifles have the letters JM in a oval stamped on the barrel, the newer rifles have REM stamped on the barrel beginning in 2007. In 2007 Remington bought Marlin and the quality of their lever action rifles drastically dropped and then slowly improved, then they stopped making them altogether. Marlin/Remington recently started making the 1894's once again but it's too soon to tell how the new ones will stack up... With Marlin 357s there is also the issue of Micro-groove vs Ballard rifling. Micro-groove rifling has many little grooves and lands and Ballard rifling has fewer and larger grooves and lands. Older JM 1894 Marlin 357's have microgroove rifling which is supposedly less accurate with lead bullets then with copper and the newer JM as well as REM Marlin 357s have Ballard rifling which is supposed to shoot lead better... Your millage may vary...
My advise?
Getting an older JM Marlin in good condition is probably your best chance for a trouble free rifle. Next best chance would probably be a new or used Rossi.. The next option would be to try a brand new REM Marlin, it's expensive and carries some risk but at least it has a warranty. The absolute last thing I would recomend to someone not very familiar with lever actions is to buy a
used REM Marlin, you might get lucky and find one that is OK, some are, but you also stand a good chance of buying a lemon......