I am running a dual-boot T420 with Windows and Ubuntu with a 55++ battery. In Windows, when you switch between "Always fully charge" and "Automatically optimize for battery lifespan" in Power Manager > Battery > Battery Maintenance, is that selection stored in software or the battery firmware? That is, will your choice affect charging behavior in Linux?
I've dual booted Ubuntu with Win XP or Win 7 on T43, T60 and T61. I do believe battery management is handled by the OS so battery management will be different in Windows and Ubuntu. Under Ubuntu, see System > Preferences > Power Management (in the older distros I use). I imagine this applies to your T420 and regardless of which Ubuntu distro you are using.
My experience (and conventional wisdom) is that battery hours are better in Windows than Ubuntu. However, if you google something like "extending battery life Ubuntu" you'll find tweaks not available in the Ubuntu Power Management preference.
I haven't installed the latest distros (this is written on 10.04.4 LTS) but I have noticed comments that newer distros provide better battery hours.