difference between sap ce 7.1 and pi 7.1

hi

i am trying to find out the difference between CE 7.1 and PI 7.1. i guess both of them has got the basic foundation to develop webservices. then what more advantages we get in pi .1 being not a free downoadable version on SDN.

and also i want to go through some business requrements which we can meet with pi 7. please refer any web links

cheers

akhil