What do you primarily use your system for, and what do you hope to gain from an SSD upgrade? If you game, and want to improve load times, then in addition to your OS and applications, your most-played games should also be installed to an SSD, in which case you might want to look for something with more capacity, probably at least 500GB to store a decent number of modern titles.
While NVMe drives will technically be faster than SATA drives, they won't improve load times much in most real-world scenarios, and tend to cost a lot more for a given amount of storage. So a decent-sized SATA drive like a 500GB Crucial MX500 or Samsung 860 Evo might be preferable over a faster but relatively small 250GB drive that can barely hold anything.