Here's a little info from Taco on Delta T vs Delta P circulators. Each have their merits and this article is a little slanted toward the Taco "way" but it contains a lot of good info. Just thought I'd share. An educated customer is my best customer.
http://www.taco-hvac.com/images/vdt_resources/WhitePaper_00VDT_DT_DP.pdf
http://www.taco-hvac.com/images/vdt_resources/WhitePaper_00VDT_DT_DP.pdf