The .30-30 is equally flexible as far as velocity goes – it can be loaded down as far as the .45-70 but can be loaded to considerably higher velocities if desired. For example, Alliant’s 2002 Reloaders Guide lists a 125g Sierra bullet at 2630fps. I use Hodgdon data and am running the Speer 130’s right at 2500fps.
Where the .45-70 shines is in pure thumping power. A 540g bullet, even at modest velocities, will put a serious hurt on the target. While I don’t shoot anything that heavy, I have shot 500’s at 1554fps and the recoil is surprisingly mild. Such loads are the second-best penetrators I have ever seen. The BEST penetrators, for what it is worth, are 300 and 350g hardcast plinking loads at a nominal 1130fps. These loads are extremely cheap to build, very accurate and generate about half the recoil of standard .30-30 loads. My hunting loads push a 350g North Fork bonded bullet to 2183fps and are awesome on game. Recoil for my loads ranges from half that of a .30-30 for the plinkers to over twice that of a .30-06 or 7mm Rem Mag for a 460g hardcast at 1812fps. There is a reason they say the .45-70 goes “from mild to wild”.
The .30-30 with 170g bullets are adequate for most game out to 200 yards - even in factory loads like the Winchester Power Points, which my rifle loves,. Most of the elk I’ve taken in the last 25 years would have fallen easily to such a load. The 150g bullets shoot a bit flatter but don’t deliver as much downrange energy.
For smaller deer and antelope, for widespread availability of inexpensive factory ammo, or just for plinking clay pigeons at 200 yards and beyond, I have to like the .30-30 better than the .45-70. For elk at 200 yards, for big critters that fight back, shooting reactive targets or just busting rocks with cheap plinker handloads, I like the .45-70.
The question is not so much which one to get, as it is which one to get FIRST.