Good question & good example of the irony of too many choices-
I've been doing lots of bird photography including a lengthy stent @ an eagle nest. I currently have half a dozen TCs - 4 that are Nikon mount. (I consider ebay a reasonable long term rental agency & will eventually be selling many of these.) I've done lots to practical & resolution chart testing. At the eagle nest, when the chicks were first born, I even stacked a 2x & 1.7x TC on a MF Nikon 400mm to get the shot. It was ok but certainly not great. Here are some general conclusions but there are exceptions to all of these:
-You'll always get better results from a lens alone that is the right FL w/o a TC.
-1.4x TCs will produce the least IQ degradation and are well worth owning.
-w/ very few exceptions, 2x TCs produce significant degradation & should be avoided. But sometimes they are the only way to get the shot w/o severe cropping.
-Many 3rd party 2x TCs are as good as name brand in the center but worse at the edges.
-Nearly all tele zooms have too little IQ at the long end to tolerate a 2x TC.
-An exceptionally sharp prime tele can yield satisfying images using an exceptional 2x TC but you'll still be able to see the degradation.
-1.6-1.7x TCs come close to the performance of good 1.4x TC but there are few & they are specialized for specific lenses. I modified a Nikon TC-17E II to fit a AIS tele.
For old Nikon prime teles, the TC-14B & one of the 2x TC-300s are terrific. But the 300s only fit really long teles 300mm & over I think - you can check what fits what following the links over here:
http://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/teleconverter/index.htm
Hope this helps,