Reading the below, I’d probably go for the W520. Both series are the final ones having a 7-row keyboard.
I recently purchased a used Lenovo Thinkpad T420 laptop to replace the Lenovo Thinkpad T400 I was previously using as my main platform for researching open-source network simulators and emulators. …
Source: [WayBack]Lenovo Thinkpad T420: Another excellent, inexpensive Linux laptop | Open-Source Routing and Network Simulation
Do not get the T420s as it is the slim-line version which is harder to upgrade and for some users gives thermal issues.
The T420 has only USB2, the W520 has USB3, better screen, faster processors and allows more memory (32 gigabyte for quad core processors).
Related:
Reminder to self: see if I can order this:
The Wiert Corner - irregular stream of stuff 