I went back and forth on this for my T43 as well, even looked at an X41's 12" screen. In the end I went with the 15" because I like a lot of screen real estate and while you don't have to use all that 15" offers, you can't use more than the 14" offers. The difference in weight is not a factor for me. If weight was an issue I would have gone with the X41, but I found that screen too small to put windows side-by-side, which is how I plan on taking notes.
As for screen resolution, my T43 has SXGA resolution (which is something like 1480 x 1050). I have used 1280 x 1024 for the last three years on my desktop and the smaller resolution is not that big of a deal. The fonts installed are larger than you would use on a desktop (for things like icon names and menu choices) so I have no problem seeing those, and when I type I either increase the font size or zoom in on the doc to over 100%, but I really don't need to do either. Also, most laptops (including the T43) come with some kind of zoom feature, like Fn + spacebar, that allows you to magnify the screen instantly if needed. But, again, I don't find it necessary at all and appreciate the extra real estate the higher resolution provides.