Compatibillity may be a problem with Sigma converters because the front element sticks out a little, therefore the lens has to have the rear element recessed into the lens body to make room for it. I would also check the Nikon TC for physical compatibility with your intended lenses.
I have no experience of Nikon TCs so can't advise on them, but optically, the Sigma EX 1.4x is better than the Kenko Pro. Both are better than the Jessops version which I believe is a rebadged Kenko standard model, i.e. not Pro.
Another point, if you're considering a 2x teleconverter, most of the time it'll be a waste of time. To get any benefit you have to have top quality glass in front of it, which usually means an OEM prime, in which case you'd hardly be looking at Jessops.