The 601 body is different than the 661 body. The 601 is a polymer/plastic body that makes it one of the lightest 60mm spotting scopes
on the market. The 661 uses an alloy body and also has a rotating tripod
collar not found on the 600 series. Optics
on non-ED 601 or 661 are excellent. You won't notice a great deal of difference, visually, between these and the ED versions until you crank up magnifications above 40x or so and then the ED will give slightly more resolution via better color correction. Notice I said slight. The difference is there, though, if you look for it. Very little difference between a 601 I tested and my ED 663, for instance.
Myself, I would choose the Kowas simply because of their super 20-60 zoom eyepiece. It is a significantly better zoom eyepiece
than the Regal zoom, optically and mechanically, and it has much better eye relief than the Nikon
zoom. As a birder, myself, I much prefer zooms and that Kowa
20-60 600/660 series zooms ranks right up there with the best. If you prefer fixed power wide angle eyepieces, though, you have options in either the Koaw or the Nikon
and even more options with the Regal, since it can use standard telescope
For digiscoping, I do recommend an ED scope, for the same reason as using an ED camera lens. It will get you better pics, assuming decent optics
and, yes, the Regal qualifies, here. If the digiscoping is just a sometimes thing, though, a Kowa
601 with the 20-60 zoom will work. In a pinch, I've taken nice pics with my Kowa
663 using that zoom eyepiece. As zooms go, it is a digiscoping friendly eyepiece. Can't say that for the Nikon
zoom. The zoom eyepiece
on the Fieldscope is too short on eye relief to do a good job of digiscoping.
As for a tripod on a 60mm class scope, the Bushnell Advanced Titanium 784040
is well made and sturdy, yet fairly light. Tough to beat for the money.