I have an 1974 870 with the manual 8-speed. I have been told that very little can go wrong with the manual 8-speed and will run "forever" and others have said they wish they did not have powershift. As I recall, the 770 to the 1175 had powershift as an option while the 1270 on up had powershift as standard equipment.
I have about 3800 hours on it and nearly all of those hours came from pulling a 4/16 plow, a 15' disk or field cultivator. I live in southern Wisconsin with heavy soil and many hills. So far I've never had a problem.
My dad bought the tractor new and I away's told him he should have got the powershift option, but now after reading about problems with powershift on this board, I think my dad made the right choice.