Climb to Kaiser is tougher. The majority of the climbing is front loaded. So you basically end up paying for it, if you go out too hard. Kaiser pass is the halfway point and the majority of climbing is done at this point. Its not like you get a free ride all the way back to Clovis. The grades are steeper too. Also when I did it, it was incredibly hot on the way back down and once you hit the flat lands. People where just sitting at the side of the road in shade because it was so hot. I kept pedalling but it was like a death march, because the heat just sucked all the will out of you to put any power down on the pedals. The death ride in comparison has the climbing and descending more evenly distributed. You do a lot of climbing then you get a big pay off (recovery) with the descent. So even if you blow up on the climb, you get some recovery on the way down to the next pass. There is also more people to work with on the Death Ride given that the event is much larger. Also Climb to Kaiser is treated as a bit of a race for some, as times are recorded and posted. So this gives some people extra motivation to go faster to see how much better they can do over prior years. The Death Ride, you sign your name on the poster and no one knows how long it took you. They are both great rides and very well supported. But I would say based on having done both that Climb to Kaiser is harder.