So Let's define the difficulty instead of saying which one easy.
High rounds to 100: Easy Solo
Co-op to 100 is almost impossible although I have seen people reaching round 100 in KINO spending countless hours/days.
Dealing with number of zombies at a given time: Co-op is easy
Zombie spawns are distributed in co-op for 2-3-4 player compared to solo. Therefore you have to deal zombies spawning everywhere as you kill if you don't know how to kite them or group them before you take them out in groups which is the commonly practiced strategy for high rounds unless you are using traps. This can be a bad thing for co-op for a small map like NDU where there is not a lot of room to train zombies and it is chaos everywhere even though there are more players.
Getting Revived: Co-op is easy.
Like Chopper mentioned even with half decent teammates your probability of getting back up is much higher in co-op games repeatedly unless it is a combination of very high round and a tough map. Eg: Round 57 2 player on origins with panzer on your ass and your partner is down. Solo restricts you to 4 downs and you are out.
Time: Solo is easy.
Solo high rounds are generally faster to reach than co-op for almost every map. We all know how long co-op games take. People have gotten Round 50 games under an hour for solo where as co-op games take several hours to complete.
Strategy: Solo is easy.
There are certain strategies that need to be excelled in order to combine zombie hordes before taking them out. Eg: Call of the dead strategy using VR11, Trap strategies for Five as there are no high round insta-kill wonder weapons. So definitely co-op is tough on that. COTD solo is an exception for solo as there is no insta kill wonderweapon for solo.
Lag: Solo is easy
Most people encounter lag in co-op games early than in solo. I have occasionally seen lag in call of the dead and some other maps specially in black ops when you reach 30 plus rounds for co-op. I don't remember if this is the case for black ops 2. Although someone pointed out that lag is directly related to usage of box, more you spin the box, more lag you get. This is why lot of high round people stick with effective trap strategies or rely on max ammo drops during boss rounds instead of excessively using mystery box.
Boredom: Co-op is easy.
We all know co-op can be played with friends and randoms, so there are things happening spontaneously example someone going down and being revived, although it is challenging to revive, it keeps you on your toes and it is less boring. In solo, even if you are a good player it is dead boring to repeat the same thing over and over again.
Based on some of the above things you can conclude what is easy for you.
Sorry about the poor structure of the post. I had a heavy lunch and need to stay awake.