I'd say they are both worth about the same. The 1887 is in better shape and the 1878 is older (the first year) and has a lower mintage. By the way, the 1878 appears to have 7 tail feathers (2nd reverse) from the pictures. I would probably take the 1878, but I don't think either is a "better deal."
Good move ! If you let me know what you're looking for, I may be able to help you out. I come across a lot of nice Morgans for reasonable money.