I assume you're talking about 1 TB SATA vs 500 GB NVMe. The answer is it depends on what you're doing. NVMe has garishly better large sequential transfer rates, but since that's hardly ever the bulk of the time doing most operations, it usually doesn't matter. NVMe also does better with lots of simultaneous I/O requests outstanding, which again is relatively infrequent and usually doesn't matter. The other consideration of course is that if you need 600GB of storage, having 500GB of infinitely fast storage isn't going to cut the mustard.
My vote is for a 1TB SSD because you have 2 times the space. NVMe is nice and you will see performance improvements in some specific workloads however you are probably better off with a larger SSD unless your workload can make good use of the faster speed.
Honestly, you won't notice any performance difference. I have two NVMe drives and the only time I see a difference between them and the SATA SSD's in the same system is when I copy files between them. If you are going to use it as the only drive in the system, go for the higher capacity SSD.
The ease of doing new builds with m.2 interface whether SATA or NVME can't be overlooked. M.2 form-factor all day long.
Now the question of NVME vs Sata III protocol - as everyone has already said, there are only two reasons in my book - 1) I have cash to spare in the build and can bump up the drive to nvvme or 2) the work / use-case can benefit from that extra speed (most don't).
500gb of space can get cramped pretty quick if it's your only computer - go for 1tb just to make your life easier. If you're [H] (and a little crazy) like some of us with a bunch of different rigs scattered around the house used for different purposes, you could make do with 500.
Thanks for the first hand experience folks. I almost shelled out some cash for an NVMe upgrade to replace my perfectly fine SSD OS drive. Now I saved some money. Damn these BF deals.