It's been a couple of years but the Junction road was a LOT better than the coast through St. Thomas. Hard to choose if you can only do one, though, both were gorgeous in different ways.

Here's a little video I took on the Junction road...

https://www.youtube.com/watch?v=MoD_DivqUvo

The coast road just had a lot of potholes but was otherwise pretty easy. Far fewer trucks.

A tip - now that google allows you to download offline maps, you can do that for Jamaica (download it before you go) and then use your phone in airplane mode for navigation. That's what I would do in Kingston.

That said...you could fly into MoBay then take the north coast highway to Porty - it's very easy and nice and new almost all the way so a nice soft intro to driving. Then take the south coast route through Kingston to go to Negril when you're rested and have a whole day and have been driving for a few days already.