I have the 8" spiral and 6" straight. IMHO the spiral is worth the
money for two reasons:

1. Easier to manage the cutters (replace, adjust, etc).

2. For face jointing wide boards, the machine "feels" smoother, which
for me means a gentler hold, less stress on me, more focus on
making the wood do what I want, etc.

As for surface quality, it's a close call for common woods. The
straight leaves a washboard, the spiral leaves the occasional (and
mostly invisible) fence-parallel lines (where the corners of the
cutters are). I think the spiral results are better, and easier to
clean up with a scraper when needed.

The 8" spiral seems quieter, but that might be because it's bigger and
not because of the spiral cutter.