I don't think either way is the best 100% of the time. Just do your homework and pick the one that seems to work best for that particular trip. I have got super deals from my Carnival PVP and she is extremely helpful with any issue that may arise. That said she does not always have the best price.
Carnival Liberty New Year's Eve 2007
Liberty of the Seas 5/2008+11/2009
Allure 1/16/ 2011
Vision OTS 2-14-2015
Freedom OTS 3/6/16