hi
As per the datasheet aval both the card are similar except u have the upgrade option in the WS-X6148-RJ-45 module to the PoE DFC.
But the later module comes up with a PoE DFC inbuilt..
WS-X6148-RJ-45 : Cisco Catalyst 6500 Series 48-Port 10/100 RJ-45 Classic Interface Module; upgradable to support Cisco Prestandard PoE Daughter Card (part number WS-F6K-VPWR=) or to IEEE 802.3af PoE daughter card (part number WS-X6148-45AF-UG=)
WS-X6148-RJ45V : Cisco Catalyst 6500 Series 48-Port 10/100 RJ-45 Classic Interface Module with Cisco Prestandard PoE Daughter Card (that is, includes daughter card [part number WS-F6K-VPWR=])
do refer this link for more info..
http://www.cisco.com/en/US/products/hw/switches/ps708/products_data_sheet0900aecd8017376e.html
regds