Just a little correction - in Converter terminology V2V implies a powered off source VM, though from user's point of view it may not be so.
Both approaches have their pluses and minuses. V2V is non-intrusive to the source and has practically absolute guarantee for data consistency.
P2V also guarantees data consistency but through VSS. It reduces drastically the downtime and has a broader application range (e.g. you can't V2V a Hyper-V 2012 source).
I suppose in your case you could try V2V.