I have a Bernina 880 which has been upgraded to a plus. The dual feed helps when I am putting rows together or when I am creating a block which has lot of sub units that need the seams to come together accurately. The feeding piece that fits into the center at the back of the foot meets a feed dog underneath kind of like a walking foot. It keeps 2 layers of fabric feeding thru evenly. And I have mostly purchased feet that accommodate the dual feed. However, I did find when chain piecing if I didn’t keep the pieces pretty close together so there wasn’t too long a thread chain in between the dual feed piece comes down onto the feed dog and cuts my top thread. But I love it for the things I mentioned.
I recently learned the walking foot is designed for when you have batting between the 2 fabrics. I have gone back to using it when attaching binding and find that worked much better for that process.
Your choice probably depends on the type of piecing you enjoy. If you are like me and seek out complex patterns or your own designs are very detailed and accurate matching of seems and not cutting off points is really important to you and then the dual feed will make your quilting experience more enjoyable. If that doesn’t describe the type piecing you enjoy, it may not make that much difference for you. I love mine and keep learning and thinking of new things to do with it all the time. So upgrading may give you opportunities to try something new. If you decide to upgrade I would suggest going for the best model you can afford so you can try new things. At age of 70 and a lot of different machines under my belt I have never regretted an upgrade. This will be the last machine I ever own and as I wrote in another post I will take it to my deathbed and no one can take it til they pry it from my cold dead hands. That said I can also be very content chain piecing the simple parts of blocks on my trusty featherweight.