RE: Whats harder?
First I've only done electric, no nitro or gas.
A lot of it comes down to psyche / personal skill. I have done both and for me and I do much better off-road. On-road is very competitive, traction today is basically infinite (with chemicals like paragon / tire tweak / death grip) and the emphasis is on driving a perfect line and doing so as fast as you can. Any half-decent driver can make it through the race without a major mishap so once you get past the newbie levels conservative driving is not going to win. You have to push the limits but not to the point where you screw up or lose more time with a major mishap. Tuning and power is pretty important since traction is basically infinite.
Off-road is completely different, breakage is more common, people are racing things much closer to stock RTR, and traction is so limited that the emphasis is on smooth, reliable, conservative driving. One bad jump can end the race for you right then and there. I love the races with longer >5 minute mains, like the 10 minute mains or the 20 minutes that is becoming more common with LiPo.