You can check www.socketmobile.com for more specific information, but here are the basic differences

Model:

E: entry level scanner, linear cmos imaging based.  It is more like a camera and needs light and stable hand for successful scan.  NOT recommend for book seller
M: Mid range, class one laser scanner.  This one is fine and should work in most cases.  We recommend
P: class two laser scanner.  Suppose better than M under strong day light or hard to read barcode.  I did not do any testing on those, so do not know the difference worth the notice between M and P

3E, 3M, 3P are scanners insert into SDIO slot
5E, 5M, 5P are scanners insert into CF slot.
7E, 7M and 7P are Bluetooth scanners