I have he IFLY Boeing 737NG in use and the PMDG 737 not. The main difference is the Performance. Under FSX, DX-10 Mode with Steve´s Scenery-fixer I can fly into New York La Guardia von Drzewicki Design with his New York-Scenery on. I have a I5 760 "unmodified" with a medium ATI graficboard in use. On such a thing the PMDG 737 you can´t fly this with 20 fps and need an I7-system with a big graficboard.
The normal procedures in the IFLY Boeing 737 are the same as with PMDG and the flying characterisitics are also good. Inside in the VC the PMDG looks better, but this cost performance. The Ifly has not only a VC, and also a 2D-Cockpit, wich is in some situations better to handle. The IFLY Boeing 737 is much underestimated because of this PMDG-Drummlers...
But... this is only a "private statement".
You can find at avsim a review.
http://www.avsim.com/pages/1111/iFly/737.html