Ok, so I have both now and since this question seems to come up quite a bit, I thought I'd do a "formal" review. I think the way I compared them worked well...I am working on a sleeve and used to small diameter knitting on two circulars method (both US8). This way, I k2p2 for 30 with Boye then in the same round k2ps for 30 with the Denise. Here are my findings however, it is only my humble opinion and not meant to do anything but give information! Please jump in if you find any of the points to be different with your knitting.
The needles are shaped a little differently at the tip. Hopefully you can see that in the close up photo. Didn't notice a difference in knitting.
The join...this is where I think the main issue may be. You'll notice on the picture that the join on the Boye is a screw in, curved, smaller diameter and inflexible metal, where the Denise is a smooth resin same diameter join with a "click" system join. I found the join on the Boye to be annoying in a couple of ways...
a) as I knit :XX: , the join unscrewed. You can see it in the close up picture. Every couple of rows I had to screw it back in. It wasn't enough to lose the join completly, but I think it could eventually fall out while knitting if you don't tighten. This would be much harder to do on regular circular knitting, but ok if you are using them for straight. I have never had a problem with the join on the Denise yet, but I know KellyK is experiencing one right now and has a call in to the company.
b) I don't like the curve of the join in comparision to the straight join of the Denise. I found my stitches were harder to get over that join and they didn't come up as smoothly as the Denise.
c) The Boye join makes it feel like a longer needle. Again, I really didn't like that, but it could be a plus for some.
The Denise is designed a bit more like the addi turbos (which people seem to love) in that the join is smoother.
As far as the metal vs resin, I didn't feel a discernable difference here. The stitches flowed equally well over either needle once the stitches were past the join.
I can't pinpoint why, but it seems like the Boye made me "fatigue" more. I'm not really sure exactly how to explain it. For some reason the Denise were more comfortable for me.
The cords on the Boye are a lot less flexible than the ones on the Denise.
Hope this helps. I know there are lots of people who love both of these sets. Denise's are ususally cheaper, so that could be a deciding factor unless you get a really great deal on the Boye.
PS The knitting on two circulars is pretty fun. I like that you don't have to worry about the stitches falling off a dpn and Amy's video, as usual, explains it perfectly.