I think the TR77 is just fine. I have a few guns that are basically identical to it except for the wood stock and have squeezed as much as 18.5ftlbs from them with the oem coil spring. Tuning is usually just fixing the guns flaws, like a leaking main seal, replacing sticky grease with non-sticky, change the barrel washers from plastic to fitted brass to minimize barrel side play, removing burrs and sharp edges, but you can go further. For example you can smooth the transfer port, make the main seal a perfect fit for the gun, and if you have a coil spring you can drastically reduce the spring twang. That twang is what drives me nuts so I always fix that, plus the it reduces the overall noise of the gun quite a bit. You can do a little work on the barrel, properly setup the scope, and make the trigger better which is probably the #1 complaint with Crosman break barrels.
I'd tune it for sure, but it depends on your interest level and mechanical level. Most people don't tune because they're afraid to take the gun apart, worried about voiding the warranty, or simply aren't interested enough. So it's up to you, but I have a guide with pix to give you an idea of what's involved. You can do some or all, it's up to you.
The nitro vs spring is basically like you described, a different flavor. Most everyone prefers the nitro since there's no spring twang. It's up to you, but you really need to try them both to decide. If your gun is accurate you might think twice about trading it for another because you might not be so lucky next time.