But to go off your list here a bit I'd say bump up to the T430. i7 versions can be had on eBay for $250.
If you have an attachment to the keyboard on the T420[1] you can easily buy the keyboard and put it on the T430 (I did this on my x230 from a x220).
Also if you become interested you can neutralize the Intel Management Engine and install something like coreboot and sea bios.
[1] http://www.thinkwiki.org/wiki/Install_Classic_Keyboard_on_xx...
EDIT: To take it further you could even upgrade the CPU to a i7-3632QM which is a quad core.