It depends on what you want. If you’re a basic computer numpty, than you should use Shopify. If you actually know what you’re doing, than you’ll find Nop is faster, easier and gives you a lot more control.
I would phrase this a little differently.
If your a basic computer numpty, (who knows about ecommerce, sales and marketing, and your goal is to make money rather than program) then you should use shopify.
If you actually know what you're doing (as a programmer), then you'll find NOP is faster, easier and gives you a lot more control (as a programmer, it also gives you a TON of responsibility of things you will have to monitor, update, fix, renew, correct, patch, troubleshoot etc).
Keep in mind, if you ever want to be successful with e-commerce, coding, will most likely not be part of your daily routine. Successful eCommerce people focus primarily on Sales, Marketing and bottom line profit, that is the core of your day, if you're spending you time fixing something with your code your neglecting another part of your business.
NopCommerce is definitely a a cool tool for programmers, and certainly you can customize NOP to do just about anything but, you need to either have a programming department which will undoubtedly cost far more than Shopify or almost any hosted platforms most expensive plan, or you're going to have to spend your time doing all the backend stuff and less time focusing on the most important part of your business, driving sales.
I know alot of pro nop people (who are programmers) tout the things that nop is capable of, which is everything since you can obviously custom program anything, which is great an all, but in the real world how many big NopCommerce sites are there that do NOT have their own programming department? Vs. how many multi million dollar websites are out there that were started by grandmas, stay at home moms, and high school drop outs and just general people with no programming experience?? There's a bunch.