Sorry misread your post before i posted, though DSO was your main use. but I'll leave my response as is.
Data rate for long exposures not so critical. If your laptop is using a HDD instead of SSD that becomes a speed bottleneck. If you want to shoot planets, solar or lunar the data rate may affect things. Often though what apps you have running will cause problems, especially realtime virus scanners trying to scan your video as its being recorded. I havent used my 120MC for deep space objects, mostly capturing a cropped region for planetary and fast frame rates. But full res for solar and lunar hasnt been a problem for me on usb2. I'm tempted to say usb speed will never be a problem for you, my experience is the recording drive and pushing the camera frame rate to its limit.
Still 3 is better than 2 right?
The 120MC cameras can also be used as a guiding camera, I dont have experience here so dont know if usb2 vs usb3 can improve guiding capabilities if you repurpose it later on.
Either way it is a good camera and it seems no difference besides usb between them. I stuck some chip heatsinks on mine when I got it. Made a small temperature drop in firecap, last week I ripped them off and slapped on a peltier, yet to see light with it. But the rear being flat (no buttons or connectors) make it an easy camera to experiment with cooling if you want and the aluminium housing stays cool. I really like this little camera, simple to use and good results without breaking the bank. ZWO seem to have a bunch of options with cooling built in, if you can afford one of those it might be a better option for low noise long exposures.