my $0.02: I would go for T420s. - you don't really need IPS for development. - you can swap battery easilly in T420s and you can get additional in the ultraba

my $0.02: I would go for T420s. - you don't really need IPS for development. - you can swap battery easilly in T420s and you can get additional in the ultrabay if battery life is really big concern. (in any case always get a new battery if you are buying second hand laptop)