I think the sl might be better than the T series, in the sense that it has HDMI.
In the sense that it has HDMI, yes, it is.
The other thing in which I like the SL400 more than the T400 is the batteries. T400, like 14" wide T61, uses the special widescreen batteries, which aren't compatible with any other model, and stick out a whole lot at the back of the laptop (esp. 9-cell). The SL400 uses the standard batteries, with the 6-cell being plush, and the 9-cell sticking out less than the 6-cell on the T400 (like WTF?).
In everything else, T series is better.
What thinkvantage tools are usable on a sl400, and what is missing?
This is actually a very interesting question, for which I would like to get an answer. According to this Lenovo page
, almost everything, if not everything, is supported.
What am I going to miss most that my T40 has? Is the keyboard\ergonomics\ultranav as good as a T series?
Most likely you will notice that the notebook is thicker and heavier than your T40. As far as ergonomics go, the peculiar angled design of the SL series caused some people to complain that ports are not as accessible. Finally, the keyboard, while seemingly still has Thinkpad quality (and in all honesty, probably cannot be worse than the awful keyboards on the T400), has a different, an IMO, worse layout - with PgUp/PgDn relegated to where the back/forward keys are on a normal Thinkpad, away from their original classic positions in the 6-key block at the top.
As far as performance goes, rest assured it will be a huge leap forward from your T40, and if you get the high-res screen, that is going to be a very nice bonus too from your current XGA.