There's no way that changing the entire roof will be easier or faster than just changing the skin.
To change the skin, all you have to do is drill out the spot welds and pull the roof skin off. To put it back on, especially if you have a stock donor, you just need clamp the skin down to channels and inner structure and spot weld it back on. Invest in a good spot weld cutter and this won't even take a full day. You don't have to worry about the body or roof going out of square, and the welding is super fast since you're just doing spots. And when you're all done, it will be nearly impossible to tell it was done, since it will be the same process as the factory.
To change the roof, you need to brace the entire body to keep it from flexing. Once that's done, you'll need to cut the roof off. This part may be faster than drilling all the spot welds, but keep in mind you'll want the cuts to be straight and level so the new roof will fit. Once that's done, you'll need to position the new roof in place. This will be harder, because the roof will flex and because you won't have nearly as many good clamping spots. Finally, you'll have to do all the welding. You'll have a lot more welding to do, as you're going to have to weld around all of the pillars. Not just one side either, you'll need to go all the way around the A pillars, not to mention the sheet metal on the B pillars on the outside, and then the inner structure on the inside. When you're welding the sheet metal you'll have to be very careful, as you have lot of ground to cover. Plus, you also have to line up the edges. If you didn't cut straight, you'll have gaps, and filling gaps while welding sheet metal is pretty much impossible. So, not only are you doing a lot more welding, but you're doing work that's harder, and much slower, to do. Finally, now that you're done with all that welding, you'll have to grind the welds smooth to get your windshield to fit properly and so the welds won't show when you paint. This step is completely unique to changing the whole roof, since if you just do the skin the spot welds are hidden in the factory locations.
I've chopped a top before, its a lot of work. The skin will be a TON easier, faster, and will have a much better chance of turning out right. I'll be doing this exact same thing on my bronze dart. I have a donor roof too, and there's absolutely no way you'll catch me changing the entire roof.