Probably depends when you are riding relative to traffic. I've ridden up Brea Canyon as far as Pathfinder, it was fine on a Sat morning. But I'd guess on weekdays it might be busy with people avoiding the 57. Corona route uses SART which is nice in that area. Probably flatter route than Diamond Bar. From Irvine you have to get to Brea to go that way as well. Again depending on timing that might not be very pleasant.