I find that if I work on a ufo, I want the same machine I started the project for. I just came back from a retreat and I took 2 machines, my Featherweight and my new Kenmore. I use the Featherweight because I had started with the Kenmore.
There were machines there from very expensive Babylock's to reclaimed machine.
As to lightweight machines the size and less features it has the lighter it is. It all about what you want to achieve at the retreat, but mostly to have fun.