SIMPLE SOLUTION: GET BOTH !!!
I own both. I prefer the .35 as it is just a bit "more". However, I truly love the .30/30. Either is all you really need in north america.
Bullet selection is very important for either to give optimum performance. However, that is made very easy.
Remington Corlokts.
Dosen't matter if you shoot factory or reload as I do.
With my .35 shooting a 200gr Corlokt over 40.0gr of H4895
(NOTE: THIS IS ABOVE SAAMI PRESSURES BUT IS LOWER THAN NOMINAL MAXIMUM PRESSURES FOR OTHER CARTRIDGES CHAMBERED IN THE MARLIN 336. USE WITH CAUTION AND DO NOT USE IN ANY SEMI-AUTO FOR .35REM.)
I find accuracy to be amazing and power adequate for any reasonable use.
Ditto the 150gr and 170gr CorLokts in the .30/30. I load the 150gr over 35.0gr of RL-15 for 2,400fps. This year I finally broke down and used the 170gr CorLokt over 34.0gr of RL-15. Also amazingly accurate! (Sub MOA from my Glenfield 30 - half magazine carbine).
I "vaporized" a deer with it on the run at about 110yds. I fired three times at the fleeing deer. On the second shot I saw a white vapor/cloud. I thought I'd hit a frosted pine tree and knocked frost off it, and missed the deer. However, the deer ran off obviously "hurt". It ran about 40yds and collapsed. I'd broke the left front leg with the first shot, hit it in the right paunch and bullet exited the upper left shoulder/chest with the second shot with the exiting blood mist causing the "vapor ball" I saw, along with a 3" dia. exit wound ! About 20" of penetration, and very similar to what I see with the .35rem. (Third shot was pulled, and deer was making the "low-crawl, mad-dash to eternity" anyhow......).
Of the 12 deer I killed this year, I took the last 4, including the largest one- a 180lb 8pt with the "ole' .30/30". It's my favorite "walking around" rifle as with a 4x Nikon scope, it weighs less than 8lbs and points/swings like a british fowling piece. It's also a very "lucky" rifle too. Of the last three "trophy" size deer I've killed, two have fallen the the "hurty-gerty". (As my brother calls his ancient (pre-'64) Win M94). Also took the largest pig I've killed with the .30/30.
Now my delimma......... Carry the .35 or the .30/30 ??????????
I'm "democratic", I take turns !!!!!!! (also with the Rem M7 in 7mm-08, usually reserved for places where shots might run in excess of 200yds, along with too many other rifles in the collection to get around to hunting every year. (It's been two years since the darling .257Roberts drew blood. It gets to go next year !).
I killed one deer with the .35 this year; and thought "oh well, too easy !!!!!"
Bang!, thud! ............. drag deer.... (Shoulder/spine shot at about 75yds.)
You really can't go wrong with either.
(I also own a Marlin 1895G in .45/70, but can't say it's any better on our "little deer". But, it does well too. Just dosen't handle as well as the little .30/30)
And BTW, in my neck of the woods, the .35 is alive and well and most mom & pop shops do carry the .35. In fact, two that I stopped in last January had sold out of .30/30, but still had several "dusty" boxes of .35. Go figure !!!!