Not having a mic honestly makes no difference between the two.
Teamspeak doesn't cost anything and people can join and leave at free will unlike on skype people would need to be added as skype friends then added to calls during a server. Teamspeak also has push a button to talk feature so you don't hear noise and other sounds from people when they aren't talking (also removes echo sounds).
So Teamspeak is definitely the prefered method for any group call. Skype is for 2-3 people max.