How about starting with the 45-70, since it's easier and cheaper to come by dies and components. There's no shortage of loading info, either. Later on, if you want, you can always have your chamber lengthened to 45-90.
If you want to go ahead and start with the 45-90, and the kick is unpleasant for long range sessions, put in less powder and more wads, until you make a 45-70 out of it. I can't tell you about kick, because it's just too subjective, and I also don't know how heavy a rifle you're planning to get.