Identical they're not. The major difference is the screen resolution:
ThinkPad W500 (4061-B13)
Based 4061-2ZU: T9400(2.53GHz), 2GB RAM, 160GB 7200rpm HD, 15.4in 1680x1050 LCD, 512MB ATI FireGL V5700, CDRW/DVDRW, Intel 802.11agn wireless, WWAN option, Bluetooth, Modem, 1Gb Ether, UltraNav, Sec Chip, Camera, 9c Li-Ion, WinXP Pro
ThinkPad W500 (4061-A51)
Based on 4061-2ZU: T9600(2.8GHz), 2GB RAM, 160GB 7200rpm HD, 15.4in 1920x1200 LCD, 512MB ATI FireGL V5700, CDRW/DVDRW, Intel 802.11agn wireless, Bluetooth, Modem, 1Gb Ethernet, UltraNav, Secure chip, Camera, 6c Li-Ion, WinVista Business 32
I've used an extremely similar unit to the latter, and hated the LCD. My pick would be the first one, for the screen alone if for nothing else.
You are *extremely* unlikely to be able to feel the difference in performance between T9400 and T9600. Put the $40 towards a SSD or a RAM upgrade....
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George (your friendly FlexView farmer)
Collecting SSDI: A31p, X24,
Abused daily: T43pSFL, R60F, X60T
On the way out/for sale: R60, R61, T43p, T60, T60p, T61, T61p, T410, X61, Z61m Ti