I think BMSbattery feels the BPM2 can handle more power as they bundle the kit with a 30A controller, whereas the Q128 comes with a 22A controller. Neither controller supports an LCD, just the 810 LED display which means you have 3 PAS levels. My first ebike kit came with an 810, and the pedal assist is a bit abrupt at low bike speeds. I'm used to it, but it scares newbies, especially if they haven't ridden a bike in years. The torque simulation controllers feel better, in part because the first assist level is pretty mild.
Bafang ships close to a million motors annually. I think it's a "big name" in ebikes because of what they also do in the mid drive market. If this was a one time buy for me. i'd be willing to pay a little more for the bigger controller, possible higher power, and "name" and get the BPM2. However, I've put together a few kits, and now I look at lower cost. I'm pretty sure a Q128H would work. I bought a bare Q128H last December ($140 shipped). Not installed yet.
For someone in the USA, the ebikeling 500W front motor kit for $209 shipped on ebay looks pretty good to me. Comes with SW900 LCD and a 22A controller. Might not be available in 700cc, but 26" is available. I bought one of those too. While also not installed, I have tested it. It runs on 48V. My first ebike kit in 2015 came from him. Same motor. Has worked fine.