Does anyone know what the difference is between these models?

Are you talking about the 990 that was made back in the early 80's? If so I bought mine in 1984 and have used it ever since. And I do a lot of sewing. Just this last year I had to have it serviced for the second time in it's entire life. As I remember it is the first computerized machine Viking ever made. I've always loved this machine and hope it keeps going for as long as I do.
I can't help you with info on the 980 but maybe Billy can. His name is Lostn51.

Thanks. I found a 990 for $75, but it doesn't have presser feet, only one. I probably won't need more than that one because I'm going to teach my kids to quilt (piece, basically)with this machine. I would love to find some feet for a reasonable price, but I guess it's not crucial. What do you think of that price for such an aged machine missing feet? Was the servicing you had done expensive?

I'm in CA so my prices maybe different than yours. It was $120 but concidering it's only the second servicing in 17 years I think it was well worth the money.
The price to me sounds very reasonable. This is a work horse of a machine. With the correct needles you can sew denim easily and even do copper punching. If you don't get a user manuel let me know and I'll photocopy mine and mail it to you.

I was disappointed to find out that the card with sides A and B for decorative stitches is missing along with the feet and owners manual. I withdrew my interest at that point. I just can't figure out how people separate a sewing machine from it's original elements and accessories. Thanks so much for your offer to copy your manual for me. Have a great day!