FYI, the results of a little sleuthing on the web.
"How does DYMO Stamps work?
DYMO Stamps software works exclusively with DYMO's top-of-the-line LabelWriter 400 Turbo, Twin Turbo and Duo printers. Through DYMO's postage partner, Endicia, DYMO Stamps users can easily establish a free online account to purchase and print precise amounts of USPS®-approved postage."
The above is off the Dymo site. So effectively, Dymo Stamps is a partnership . So you can either go via Dymo or directly to Endicia. If you select the Dymo route make shore you have the right Dymo printer ... the 300 series does not work with this service. The newer printers start at USD 110 and run out at USD 210 (plus S&T).
If you have the earlier printer, I presume you would have to sign up directly with either Stamps.com or Endicia (i.e. monthly fee plus postage).
As to the original question "Pitney Bowes vs Stamps.com" what is your need today and in the foreseeable future? Piney Bowes invented this market space and has solutions that can grow with the business. The others have only a short history and I question their ability to scale up. Do you home work before you buy!