Re: Blissey or Miltank?
Blissey is often considered 00ber, so you may want miltank. I dont consider blissey 00ber, but many do. Another good point is miltank is the only pokey that can learn heal bell naturally and breed it onto others.
But what mlugia said, explain their roles in your team, and intended movesets etc. You cant compare them well the way you asked.