My own, humble opinion is that you can just about flip a coin.
Grizzly's gotten a lot better in the last several years and Sunhill's
always had a good rep. In fact, I was in the same boat as you some
months ago except I'd decided to go with an 8-inch model.
Returning that thing is going to be a class A pain, no matter who you
buy from. That's why lots of folks go local. In my experience, the odds
that either Sunhill or Griz will send you a lemon are about the same:
really really small.
If price isn't your biggest concern, try to find out what the
differences are between the two models you like. There's bound to be
SOME, however small. Then choose the one with the features you like
I've got the Grizz G0586. Love it. Fit 'n finish is great, parts were
all there, adjustments and calibration were easy. Does a nice job
flattening wood. Thing is, I'm pretty sure I'd feel the same way about
the comparable Sunhill model. :-)