Hi Bernard,
Ever since X-rite bought Gretag-Macbeth, the i1 portion has been massively disjointed and hard to sort out. I guess to be expected with mergers... I have i1 Photo and wanted to upgrade my capabilities to better CMYK profiles. The option of buying the key to unlock that portion was buried in their site (at the time - now it's listed as an option). It actually took chatting with a rep at PhotoExpo to find out where to actually fins it on the X-rite site.
To answer your question, the primary difference is one of use. Do you plan on profiling mainly RGB printers, or do you want to create profiles for RIPS and other CMYK devices? If your answer is the former, then Photo will suite your needs. If you want/need both capabilities, then, sadly, your best option is the priciest. The most elegant soluton is ProfileMaker, but that's even more expensive than i1, but it is far more extensive, and allows much more tweaking of profiles. i1 Photo has been replaced by i1Xtreme, and is a good place to start. I recommend buying the spectrophotometer with the UVcut. (Mine doesn't have this, and profiles are slightly effected by using papers that have brighteners -- or so I've been told.)
My 2-cents.
Nemo