NVMe or M2 SSD

I'd suggest an m.2 NVMe SSD.

NVMe is a protocol. m.2 is a form factor / connector. You don't have to pick one or the other. You can pick both.

The real question is SATA SSD vs NVMe. Both will be throttled to USB 3.0 (~500MBps), however if you later upgrade to a newer mac and reuse the drive a NVMe will run faster under USB 3.1/3.2. Also these days NVMe prices have dropped enough that they are often cheaper than SATA.

Considering you're going to be using a USB-A port on a 2015 model, I don't think it matters. Buy the one that appeals to you as best value. It's performance is going to be determined by the Mac and it's usable ports, so it's a tossup ... a personal choice.

I've used both a portable SSD and a separate enclosure drive. The difference was not detectable.

I used a Samsung T7 portable SSD on my Late 2015 21.5" Retina... worked flawlessly. Made the machine a joy to use.

Th only practical ports you can use are the USB-A ports. The Thunderbolt 2 ports are virtually useless. I got read speeds in the 450 MBps range which was 5x the internal HDD's speed.

If you choose the T7, or similar, you will not get the max potential speed of 1050 MBps.

M2 is something completely different from an intel Mac. it is not possible to install an M2 chip into an intel Mac.

nor can you install an M2 chip into a stand alone enclosure. sorry.

yes, sorry about that. I'm the first to admit that I don't know everything. and padams35's reply taught me something new. :)