I looked at both before I signed up, and chose Alcor. Both organizations have similar numbers of members and corpsicles, but I bet the average wealth of their members is quite different. Alcor's higher dues are reflected in their staffing, research, and legal battles. CI is much more low-key.

The biggest difference between Alcor and CI is that Alcor does standby and transport. If you're very ill, they'll send a team to your deathbed so you can be cryopreserved as soon as possible. If you go with CI, you have to contract with Suspended Animation to get that treatment.