The biggest difference between the Tecra's and the Portege's is size. The Portege line is Toshiba's ultra-portable line menaing that the tend to me smaller (thereby more compact) and ligher (the S100 weighs 1/2 lb less and is 1/4 inch thinner than the M3). as far as ergonimics, since the rest of the dimensions seem similar I would imagine that the keyboards and stuff are going to be pretty similar in size and layout, although the keystrokes are shorter (meaning you don't push them down as far) in the S100, which could be either a pro or a con in terms of ergonomics (less effort in each keystroke, but also easier to accidentally hit extra keys). Also the M3 has dual pointing devices while the S100 only a touchpad. While the touchpad seems to be more popular among notebook users, the location of the stick (right betwen the 'G' 'H' and 'B' keys is very convenient in you want to move the cursor while typing without moving your hands too much. But, if you haven't used the stick before, it does take some getting used too.
The Tecra seems to have more ports on it (at least SVideo out, Parallel)
As far as your question about the differences in the graphics cards, the short answer is yes, the one in the Tecra is faster, but wether you will notice the difference or not will actually depend on what you will be using it for. If you will be using it for "typical" stuff (MS Office including PowerPoint presentations, internet browsing and email, and watching an occasional DVD) you probably would not notice the difference. However if you will be using it for graphics intesive gaming, 3D graphics work or similar stuff, then you will definately want the card in the Tecra.
As far as build quality, there's is not much I can say since I haven't actually seen either model in person. maybe someone else on the list can answer that later.
Hope this helps and good luck
edit: fixed to read 1/2 lb lighter, not 1/4