In short, the Crips are most likely more powerful, but it'd be hard to determine who is in fact the most dangerous. I'll go into detail below, and I don't mean to insult your intelligence when I explain simple gang knowledge, I'm just trying to make sure I give you a thorough answer.
The first Bloodz set was originally a Crips set. (A set is a division of a gang. For example, say there were two groups of Crips in a city, one on the East side and one on the West side. Members of each group would identify themselves as either West Side Crips or East Side Crips, but in most cases they would not fight each other because they are still both Crips.) The Piru Street Boys were a set that were considered to be Crips, but they began to feud with other Crip sets, and eventually broke off to form the Bloodz. Although many Crip sets are alligned with each other, they sometimes feud amongst themselves, but that is not to say that the Crip sets feuding with other Crip sets wouldn't feud with Bloodz as well.
The Crips have been around longer than the Bloodz, and as a result they have greater numbers than the Bloodz, and the Crips are alligned with the FOLK nation, therefore they are alligned with the Gangster Disciples which is easily the most powerful gang in the Midwest.
Bloodz have a reputation for being dangerous because of the violent types of initiation they use. Although it varies from set to set, Blood initiation often involves acts of violence directed at others. A common initiation for the Bloodz requires the new potential member to go out at night accompanied by another Blood, and slash a pedestrians face with razor blades. Crips usually get "jumped in," meaning the potential member is beaten for a certain amount of time by a certain number of members of the set they are joining. I'm not sure what the rules for Crips are, but when you get jumped into the Gangster Disciples, (which are the biggest gang in my city) six guys beat you for six minutes, you are not allowed to fight back but you are allowed to block their punches. The number six is significant because of the six-point star, which is a symbol for the FOLK nation, which the Crips are also under. (The Bloodz are under the PEOPLE nation, and are alligned with Vice Lords. The PEOPLE nation uses a five point star) Crips also can be "blessed in," which is simply one member giving someone permission to join without any beatings. Sometimes a ceremony is performed for someone being blessed in. I have never heard of the Bloodz blessing somebody in, but in the occasion Bloodz jump someone in they don't have set rules for the beating that takes place. Usually the potential member will be beaten until they are nearly dead. I have also heard of Blood sets that require the person being recruited to be burned on the wrist with a lit cigarette
Bloodz are also known to be violent because of the fact that most sets do not allow someone to leave after joining. Many gangs have a policy of "blood in, blood out," which means that you are in for life, or you will be killed or beaten badly. It all depends on the set though. I know that in most Gangster Disciple sets, you can "drop your flag," which means you no longer rep GD but you do not have to be jumped out. I would imagine that Crips have a similar policy.
So, overall it would be hard to tell because of the different policies that each set has. For example, the GD sets in my town don't have nearly as strict rules as the original GD sets in Chicago. But based on the many facts about each of these gangs, Crips are most likely more dangerous than the Bloodz. And personally, I would agree because in my city and most other cities with gangs that I know of, members of the FOLK nation outnumber members of the PEOPLE nation.