what are the notable differences between these two codebases? i've only used SWRFUSS that i'm aware of, and i'm curious whether SW FotE has a similar feel and what the variances are between them. thanks.

The most noticeable difference between them is the space system.

FotE features a completely redesigned space system that features prototyped ships instead of builder created ones. There are plenty of other difference, but Space is the most noticeable. SWFotE is a derivative of SWR that was popular enough to warrant it's own FUSS branch.

is either noticeably more stable than the other, generally speaking?

Both of the them are equally stable to my knowledge. But I don't use either regularly.