I generally consider these sort of comparisons loaded since Marlin is considered levergunning bedrock and the 336 has such an entrenched following, but what the hell, I'll play anyway
My old Marlin 336 Stainless (pre-Remlin)
My Mossberg 464 Marinecote (Davidson's)
I picked both rifles up used; first the Marlin, then the Mossberg later. In comparing the two, the main thing that stands out to me is the action and weight. The 336 feels more mechanical in its lockup when you bring it back to battery. Likewise, the lever safety clicks positively when fully engaged. The 464 feels more organic, conversly; reminding me of the grip safety on a 1911 pistol. The only tactile sensation the Mossberg provides is the metal to metal contact of the lever against the rifle's frame. at first it was disconcerning when running the action in test, but actual fire reveals it to be quite natural.
On the action itself, the Mossberg's 94-esque action seems smoother and more natural, though it does use top ejection which would normally create scope conflicts. The 464 remidies this with two bolt guides drilled and tapped specifically for that purpose. In fact, I would guess that the shell ejection has been toned down specifically to address that issue. At first I thought the Mossberg's ejection was overly weak, though I now suspect is was rigged so as to avoid shells pinging off the optics. The only issue you might run into is with shorter scopes and spanning the gap between mounts, a problem you simply won't have on a 336 featuring sideward ejection. It's not the end of the world, however. Offset rings are availible from a handful of manufacturers if you must have a compact setup.
While the Marlin felt just as solid and well built as the 464, I wasn't a fan of the 336 action overall. Like the lock-up mentioned above, it felt overly mechanical through the swing. It also felt heavier, but realize I'm comparing the Marlin's 20" barrel to the Mossy's 16" barrel. Even so, the specs say Marlins are still heavier when compared one to one.
If one must have a safety and both of these rifles did, the Mossberg is hands down the better. The 464 features a tang mount safety that is not only ambedextrious, but nearly impossible to bump accidentally. It's also plastic which is a bit of a turn-off, but overall it's tons better than the 336s little side button. On a side note, the 464's hammer is pre-drilled for a thumb lever and includes one standard on all models to my knowledge. Even though my 336 stainless had one as well, it's a nice touch for the supposed "budget" 30/30.
In terms of reliability, I'll be honest and admit that it's a tough sale for Mossberg for two reasons-- First, the earlier runs of the 464 had sporatic extractor issues and second, it simply doesn't have a pedigree spanning fifty years. I'm sure Marlin had its share of issues when the 336 first debuted and like Marlin, Mossberg has done its best to quash them. Neither 464 or 336 experienced any serious issues during my ownership, save those operator induced. For example, i lubed the 464 up too much, creating a condition where the shells would stick in the feed tube. Once the rifle dried out normal operations resumed. Both rifles have had shells hang up when feeding into the chamber every now and then, but that goes with the levergun territory. Normal run Mossbergs are also reported to have gritty out of the box triggers, something I can't attest to with my Davidson's special. The trigger and action are smooth as silk.
Comparing the two and realizing that such a comparison is highly subjective, I have to say I like my Mossberg ever so slightly more than the Marlin. The action appeals to me and I love the Marinecote finish just as much as normal stainless. A 16" barrel makes for a wonderfully balanced package and it's a blast at the range. I can't imagine a normal run 464 would be any worse save the asetetics and such. As long as it can maintain its reliability, I'm just as pleased with the Mossy, if not more so.