Honestly, either one would suit your needs just fine if you are going for the Guide Gun option. If you were going with a longer barreled rifle in the .45/70 I would say go for that since it would have a bit longer range capability than the .30/30. But since you are looking at basically the same rifle just different cartridges then there really is no advantage for going for the .45/70. The little ole .30/30 has downed more big game in North America than ANYTHING else. Hands down. OOPS in the last 100 years anyway. Knock down power is simply a myth in all reality. Placement is the key to dead right there results. Bullet performance is next followed up by caliber size and muzzle velocity. For dead right there, no tracking needed results you have a few shot options. Head, Neck, High shoulder spine (my preferred shot) and SOMETIMES you will get a no tracking bang flop from just a heart shot. I honestly don't know how many big game animals I took with the little old underpowerd, can't kill nothin cuzz it's just too little .30/30
Couple hundred hog, at least 2 or 3 dozen deer, several Elk and a few Moose. With todays newer loaded ammo for .30/30 such as Hornady's Leverevolution the .30/30 has an easily reached range of 200 yards nowadays. If you go for the .45/70 you are limited to the anemic factory rounds which are severely underpowerd MUCH under the true capability of the round. If you reloaded then I would say go for it as you can seriously up your game with a .45/70 making it a true performer. But limited to factory rounds it really has no advantages over the .30/30.