I’ve used this T420 as well as its successor T430 for work. With the exception of the Chiclet keyboard and updated screen, they weren’t different spec-wise.
I personally didn’t have anything against the Chiclet keyboard. I felt they were a little softer than the older one that this T420 has. Probably because of the curved nature of the keys but it had great travel was a lot quieter. Its also a heck of lot easier to replace. On the plus side the Chiclet keyboard is better at handling spills.
I’d recommend the extended battery and SSD in place of its spindle drive. Download the drivers and/or copy them to a flash drive before you wipe out the drive. The installers for drivers are sitting in a folder in root (Typically C:\IBM) in most cases.
The current gen T430 has changed the touchpad and eliminated the physical trackpad (top) buttons which makes it bigger but the purists are up in arms about that change. There could’ve been another way to have achieved the goald of bigger touchpad without sacrificing the physical buttons like making the spacebar smaller and inserting the trackpad buttons in the middle or to either side of it.