They are suprisingly different in sound. It's probably fair to say that the '79 is quite similar to the Euro 4LX I had (and sold to buy the '77) so I feel that you are right to stick to your NS-2 basses. I'm very shallow so wanted the two basses to match.
The '77 has a really nice tone to it. It's fair to say these are hot pickups so a little care is needed dialling them in. I tend to run the bass preamp EQ with the bass at around 60% and the treble at about 25% for a bit of definition and this produces a nice warm P bass tone that sits really well in the covers band I play with. I also turned down the overall preamp gain.
I think these are cracking basses and I genuinely can't see me selling either of them. The '77 is the marginally heavier of the two at 3.9 kgs (vs 3.8 for the '79) so both very manageable for gigging. I love the natural matt finish that makes the necks very special to move around on. I'd say the neck profiles are a little more P than J.
As you know the NS bodies are very comfy to play now I have managed to get rid of my 6-pack they are perfectly profiled. The vintage style frets are my preference in any case and, it may be sacriledge, I prefer the dot inlays to the Spector crown inlays
PJ told me they only made around 50 of the '77 basses but not sure how many '79s will be made, just yet at any rate.
I tend to use the '79 in the originals band but I don't have any good recording I can share for that. Here are links to couple of recordings I made with the band using the '77. Apologies for the abosolutely average playing and the shades (and the hat for that matter) they made me do it