I have been hit by the vintage bug last year and I got a nice Singer 15-91 in a cabinet. The harp size is quite roomy, and it is really fast! I'm working here on a twin size quilt, and I'm not struggling at all.
I also have a Bernina Artista 730 with the BSR, but the harp is a short 7.5, so I have found that the best combinaison is to "stabilize" my quilt with the 15-91 or use it for design made with a simple walking foot. Then I free motion with the Bernina all the area where I want a more complex or detailed design.
I also have a Featherweight (for piecing) and a old Bernina 831 (for general sewing).
My dilemna right now is to replace or not the Singer 15-91 by a modern mechanical straight sticher (Juki 20-10/Brother Q-1500/Pfaff Grand Quilter).
I must say I don't love my Singer 15-91 like I love my vintage Bernina. What I feel is more like the respect for an excellent tool, still performing flawlessly after more than 50 years. I certainly appreciate that. And each time I use it, I'm amazed by how well it sew without any fuss.
The only thing I miss when I'm using the Singer 15-91 are the "modern" feature that I'm so happy to have on the Bernina: thread cutter/ tying off/ needle up/down. I know, it's silly. But I make a lots of start and stop in my designs.
The Singer 15-91 is fast; faster than the Bernina for sure. But is it doing 1500s/m? If I compare with my serger, I don't think so. Maybe more 1000/1200s/m.
Anyway, I'm torn. Rationnaly, it seems stupid to replace a perfectly performing 50$ machine for one that will cost 500$. So why am I itching to do it?