Hey there Sharon, I don't have either of those versions of the machine but I have had and still do have a Horizon 7700. I don't know anything about the 8200 but I believe that the 8900 is the updated version of the 7700 with all the problems ironed out! I think the acufeed system is now built in - on the 7700 I have to change the feet which is a pain because they are not clip-on but need a screwdriver. The useless sliding mechanism for the single hole plate converter has been replaced with a separate single hole plate, and I think, but I am not sure so please check, that is a snap-on plate like the Bernina and most other machines instead of a screw-in plate like mine is. I have never actually tried the 8900 but I believe Pam Holland works on one and you might send her an email and ask for an opinion. I loved the 7700 for FMQ because it is sooooo smooth. That extra large harp space was nice to have too but as you know not essential and I think you have a sit-down long arm machine (Tin Lizzie?) so probably don't need the machine for that so much.
As for the embroidery side of it - the stitches are nice but I did find that the elongation is not very good and some of the stitches do not elongate at all. The variety of blanket stitches on the 7700 is nice to have but I found I mostly used the double blanket stitch most of the time which is lovely. Some of the embroidery stitches have a lovely finish but some are just okay. I have found that the quality of the embroidery stitches on the Bernina 750 is much better and the elongation is fantastic on that machine too. I believe on the 8900 the elongation is only 5 times which is the same as the 7700.
As for your 6500, they are much sought after machines here in Ireland as being one of the best and most reliable machines in the business. They were replaced by the 6600 which are also supposed to be a terrific machine but my repair-man says the 6500 is the best. I hope you will be able to keep it as a back-up machine.
Feel free to ask me any other questions for a comparison with my version if it's any use to you.