Helio, now known as Virgin Mobile contract side has the cooler phones but also has the requirment of a two year contract. Virgin Mobile prepaid side has lower prices and more freedom but lower level phones. Phones from one side CANNOT be used on the other side unfortunately.
Sprint just bought out the whole company this week. I would wait and stick with Metro for the short term. They will probably be making improvements to VM like hopefully, better customer service, wider selection of phones and better plan rates. They might add the ability to go mobile to mobile to 50 million Sprint, Nextel, Boost customers at a certain point as they integrate Virgin into Sprint. Virgin/Helio only have 5 million users so mobile to mobile there isn't as useful as it would be with all those other Sprint/Nextel people.
Sprint has been through some tough times but they have been doing well in prepaid. They also have been noted for their improved customer service. VM customer service is horrible. They are especially known for overcharging credit cards on file. Sprint hopefully will turn that around. .
I would keep my eye on VM for the near future to see what changes/improvements they are making. Right now, it's a totally new ball game and I am hoping a newer, better Virgin Mobile emerges from it all.