I would go with the tang/peep sight. If you use teh offset mount it makes the rifle IMO very clumsey to carry and shoot. Even though the venreable 30-30 may not be a long range round any offset will effect your POI at any range you have not sighted in for. If you have sighted in for say 75 yards and you shoot at 150 you will be off to the left at that range, how much I cannot tell you but it is like mounting a scope on say an AR-15's carry handle. once sighted in at a certain range the changes at diffrent ranges are much more drastic than if mounted on a newer flattop.
I have an old Savage 99 that was my Great Grandfather's, it has an original Tang Sight and even with my eyes going I love it. Something about a peep helps you focus on the front sight. Plus you keep the rifle original.
BTW - the 30-30 in "Factory" loadings is a great shorter range cartridge. It also makes a fantastic longer range handgun cartridge when loaded with 125 - 150 Nosler BTs. I have a few whitetails taken a bit over 200 yards with my Contender in this caliber. It has enough oommphh to completly pass through and destroy the lungs on those I've shot so I know it has the power. The flat points loaded by the factory loose velocity very fast.