Auto focus is a very valuable feature to have. Using manual focus can sometimes be really tough to get the sharpest focus and auto focus technology these days is incredibly accurate. With the shallow depth of field from a fast lens like that, accurate focus tends to be a lot more important as well. Accuracy is also not the only thing to consider; being able to focus quickly is really helpful too. I know you mentioned it's mostly for portraits, so you probably won't have to much subject movement but you never know.
Then there are other considerations apart from auto focus. Again I don't know much about the lenses, but with newer construction you're usually looking at a better built lens, better glass and better durability. My personal recommendation would be to make the switch. $80 is fairly small jump in price, especially when you consider the relative cost to lenses in general.
As for MF helping you become a better photographer, I don't think it's directly related but if you make the switch and decide it is, there's always the option to turn AF off. There are other times you'll be forced to use manual focus such as low light situations so you won't have to worry about become too reliant on auto focus.
Personally, the risk of missing a great shot because I don't have auto focus isn't worth $80.