To start off with I'll assume you're going to be using factory fodder, and not rolling your own. If you must go with either the Marlin in 30/30 or 35Rem., in this case I'd take the 30/30, and look to using some premium ammo for the elk, such as Federal with the 170gr. Nosler Partition load.
In a lever action Marlin my own preference would be the model 1895 chambered for the 45/70. There's a good selection of more powerful factory ammo available from several sources. Several of these hotter loadings are listed on MidwayUSA. Take a look:
Link: http://www.midwayusa.com/
As noted, another good consideration would be to look into one of the Remington pump actions chambered to 30-06 or even the 35Whelen. A friend of mine, also a "lefty", has a Rem. pump action carbine in 30-06, and it is very accurate, and makes a great woods hunting carbine.
For fast follow-up shots the pump can also be operated quicker for aimed shots than a lever (IMO).
If you want to stick with a lever action then also take a look at the Browning BLR model. I think it's even available in several magnum chamberings.