I figure the only difference between the two beings the display screen size.
T510 is better.
This is the same reason I went with the 14.1" instead of the 15.5" laptop: 15" + lcd screens ussually draw more battery power, thus lower battery life, and the bigger the screen, the more common it is to get faulty/ dead pixils/ dead screen, and the don't last as long, which means repairs- Expensive repairs, because 15"+ can cost up to 20% more the a 14".
Unless your going to to sitt in a car/ desktop/ airplane, and fiddle with a laptop while on a trip, and you arn't going to be unplugged from power very long, go right ahead and you can have a perfect laptop, However:
If you spend a long time in conference rooms/ overseas flights/ carrying it constantly, that big screen is gonna cause issues. My gateway, and its 17" Behemouth causes a square bruise in my back, and I do it so common, it has worn a 17" hole in my backpack, and I have a semi immperession in my back from it.
So: Choose the screen size that is right for your personal life...