The motor indicates how fast and smooth the treadmill will be able to sustain a continuous belt speed. Some treadmills simply can't "keep up" and the runner can move faster than the belt can go. In which case, the runner will be forced to slow down.
3.0 CHP seems to be the base for "running speed". Track stars looking to stay in from the ice may need something bigger. Otherwise it may be suitable for most people. Although, runners who constantly train at steep inclines, and/or larger runners (200+ lbs) may need a slightly more powerful motor.
Larger roller size on the other hand benefits the life of the treadmill. Wider rollers means the rollers themselves spin slower so there's less wear-and-tear over the course of a run. They have larger bearings so they can handle more pressure. Larger rollers are just flat out stronger. Then finally larger rollers have larger surface area to grab on to the belt which prevents the belt slipping (an especially annoying occurrence).
I don't know which one you should go as it's entirely based on your needs. If I was treadmill shopping, I would test out the 3.0 CHP treadmill at a slight incline to see if it struggles. Possibly even steeper incline than I would normally run so I know it's more than capable of handling my regular training. Then get the one with bigger rollers if it passes all the checks.