Btw X220 can be modded to have a 2K 16:10 display (I'm using one rn), although it's a nontrivial mod. W520 is the only model that can't (or at least hasn't) be modded at all.
I just replaced my long suffering w520 for the daily driver... with an HP z840. Yes, I intentionally stick with old business-class hardware. I rarely used it as a laptop though - it lived on the docking station and the displayport happily pumped out 4k at 29.xxx hz. No, it couldn't handle modern-ish FPS at that resolution - but it could handle the strategy/rpg games I occasionally play and I never felt constrained with the hardware video decoder. As far as mods - you can definitely hotrod a w520. The CPU is socketed, and my memory is hazy - but I remember it being very easy to exceed the stock thermals with a non-stock CPU. RAM is socketed, and exceeding the spec capacity/clock is pretty common. Various bios mods are out there. For the really adventurous, undervolting isn't that hard - because the board schematics and logic diagrams have been floating around forever.
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.