In my opinion the T61 is far superior, but there are some advantages to the T60 series.
Advantages of T61 series
Ram expandable to 8gb (T60 can only use 3gb)
SATA2 support with modified bios (T60 is sata1 only)
Faster and more efficient Penryn core2duo cpu support (T60 uses 32bit core duo and the slower core2 duo Merom chips only)
Better nVidia* descrete graphics
Disadvantages of T61
No 15 non-widescreen models
No IPS displays
Most descrete models have older design nVidia* chip that could fail
*the nVidia issue has cause prices of the entire T61 line to drop, making them a very cost effective and powerful laptop for the price.
Advantages of T60
Some 15" models have IPS display
This is really the only advantage, but these beautiful screens generally don't seem to hold up well, so if you're buying a used one, if you want a perfect screen, expect to spend a couple hundred dollars on a new (old stock) ips screen, which won't be easy to find. The generic screens sold for these are usually junk
The ultimate T61 is considered by many to be a custom built system that has the system board from a 14.1" (non-widescreen) T61 fitted into a 15" T60 chassis with 15" ips display. This will allow you to get the best screen and have upto 2.8ghz penryn cpu, 8gb ram and will support display resolution upto uxga. It's also possible to retrofit a qxga screen if you modify it's firmware.
Disadvantages to T60
pretty much everything else.
ThinkPad W-510 i7-820QM(1.73-3.06GHz) Quad Core... ThinkPad T500, T9900, 8gb SSD...FrankNpad T-60p/61p (X9000 2.8ghz) 8gb SSD ips FlexView...ThinkPad T-61p (T9300 2.5ghz) 8gb ram...Thinkpad X-61 Tablet 4gb ram...ThinkPad A-31 (1.9ghz P4 1.5gb ram)
