ThinkPad W520 is so much better while having all features of T420 and more.
Things in W520 is better:
- FullHD (1920x1080) screen instead of only 1600x900
- USB 3.0 ports instead of only USB 2.0 ports
- powerful GPU instead of limited one
- will have 4 cores and 8 threats instead of 2 cores and 4 threads
- has 4 RAM slots so 32 GB RAM is possible
In case you really need smaller size - the T420s is better:
- has USB 3.0 port
- has the same 1600x900 resolution
- is smaller/thinner/lighter
- can use UltraBay battery instead of CD/DVD
... and I would even say that even X220 is better then T420:
- even smaller
- with i7 CPU will have one USB 3.0 port
- can have IPS screen
- the only downside (and that is bad to be honest) is 1366x768 resolution
The T420 seems to be the worst of the series ...
From the daily user of FreeBSD on W520 and X220 depending on the needs.
Regards.