I think you stick with what you've got until you find out what it is you really want a new machine to do. Don't get blinded by bells and whistles if you don't think you will use them. My mom has a couple of Berninas that do everything but make coffee. And she uses the fancy stitches and embroidery functions.
I had done enough when it came to plunking down big $ (for me) to know that I wanted the best straight stitch machine I could buy. I have a cheapy brother with fancy stitches that I use only for the occasional applique, but primarily I want to piece and quilt. I have a mid-arm Juki 98Q that i absolutely love. And a 1956 Singer 15-91 that is a powerhouse as well.
Many folks use their bells and whistles, and that's great. But make sure you want to use them before you decide.
Many of us like the older singers. My Juki is much like those older machines- simple to work, simple to clean, simple to repair, and all metal. You just need to keep oiling her and she will purr like a kitten and run like big cat.