This depends on a number of factors:
Will you be adding new subdomains?
If so a wildcard cert may be for you...
Is it important that each site be secured separately?
(e.g. compromising the cert for one site shouldn't compromise all of them)
If so you don't want a wildcard certificate - you want a separate cert for each subdomain
Do you need an Extended Validation (EV) Certificate?
I don't believe any SSL providers offer wildcard EV certs...