What exactly will you be using it for?
Both cartridges are good for a trail gun. The 45-70 will kick harder, but will fire a much larger bullet than the 30-30. I wouldn't hesitate to use it on something as large as a grizzly.
The 30-30 on the other hand will kick lighter, so fast follow up shots are easier. It can easily take out animals such as deer, but I would prefer a larger gun for animals such as bear.