Which Lenovo Thinkpad should I buy? E540 or L440?

So I'm a Computer Eng. Major and i definitely want to get a Thinkpad, but also on a budget. I'm debating between the E540 and the L440. Looking to upgrade from the base model to and i5 processor as well. Any suggestions or opinions will be greatly appreciated. I actually also found a Toshiba i7 laptop but am a little hesitant to go towards Toshiba…

From what I can tell it looks like the L series is the "tougher" of the two purporting to have military specs which would make it more durable. The L440 is a 14" screen and the E540 is a 15.6" screen. Beyond that, I didn't see any real differences, but then I don't have them side by side and am looking at pictures and specs/feature lists.