It's like with that FROG discussion we had
A heavily upgraded T-55 can be a formidable foe
But if given the choice, and if you don't have any T-55s to upgrade in the first place - you might as well go with the T-72
Larger main gun calibre, more modern ammunition (I think only Israel has produced cutting-edge 100mm smoothbore ammo as of late), greater availability of ammo and spare parts (T-55 stuff is getting old and being withdrawn more and more from service), autoloader (not always an advantage if the crew is experienced, but the T-55s small crew compartment may exasperate difficulties for the loader), better armour and a probably a load of other specifics that I'm not knowledgeable about.
The Kharkov's plant T-55AGM is probably the most extensive upgrade; it even replaces the main gun with a modern 120mm or 125mm calibre.
But at the end of the day it's still just a T-55 no matter how pimped out. The market for upgraded and modernised T-55s came and went in the 80s.
The T-72 is a newer frame, and what's more it's got a lot more upgrade options. Most of the newest Russian upgrades for urban warfare tanks or conversions into BMPTs or support vehicles - are for the T-72 models. There are some that exist for the T-62 and T-54/T-55, especially from the other ex-Warsaw Pact or ex-USSR nations; but the options are considerably fewer. And the price difference won't be significant. Probably in the case of the T-55AGM; it would actually end up being more expensive than most T-72 upgrades.
Whether you gut the frame of a T-55 and replace it all with high-tech, or do the same with the T-72; the cost won't be much different.
The main way it would be worthwhile is if you already have some T-55s in decent condition along with many crews and officers experienced with them, a supply/repair network already in place for them and no threats in the vicinity that would require larger gun calibres.
For a country like Vietnam, or Peru it could be worthwhile to upgrade their T-55s instead of buying T-90s or T-72s let's say.
Romania is pushing it; as their T-55s, even with reactive armour and Israeli shells - will be severely outclassed by Russian T-90s; but then they probably don't have the money for much else and want to support their domestic industry, so it makes sense as a stop-gap at least for them too.
However, for any new buyer; the T-72 is the only sensible option. And secound-hand T-72s won't cost much more than secound-hand T-55s; there are massive amounts lying around of both models.