About gloss differential there is no possible argument. Try any luster/semi-gloss paper and the Vivera+gloss enhancer will win easily. I mean, there is *no* competition.
Canon and Epson managed a quite good performance on glossy papers, their pigments managed to retain the glossiness of the paper and so there is no much difference between printed and unprinted areas, but on papers where the unprinted area looks more matte than the printed area, only a gloss enhancer can do the trick.
Having just changed from a Z3100 to an iPF6400 I can confidently second that. The gloss enhancer is really fantastic for gloss differential.
I wonder if there is a spray, like the Lyson Print Guard perhaps, that would have the same effect as the HP gloss enhancer? And at the same time provide additional protection to the print.
In reply to the OP, my biggest wish is reliability, reliability, reliability! I think the current printers are fantastic, but all you have to do is to read some of the posts on this forum to see that there are still some big-time reliability problems with things like head clogs on the Epsons. If I was Epson I would forget about going further down the pico-litre route and concentrate on fixing the head clogs (having had 2 Epsons in the past, I didn't even consider an Epson this time round for this reason alone). I now find out that with Canon I may have to replace the print heads at a cost of nearly $1000 every year: that should not be acceptable to us. With HP the paper feed was a major pain in the butt, IMO, but my Z3100 was certainly an amazingly reliable machine (I would have bought a Z3200 if I hadn't had concerns that HP might drop that product line).