I looked at both ZeusDraw and Lineform. They both have (different) strong points and weak points. I eventually chose ZeusDraw because of the brushes. Lineform's brushes (like many of Illustrator's) work by warping a drawing of a brush along the path. If you make a bunch of brush marks with the same "artistic stroke", they all look the same. I think that's kind of boring. ZeusDraw's outline brushes have a controllable amount of randomness that you can set so the result isn't always the same. The brushes in the "brush library" make strokes with texture that can be much more interesting. Not quite like Painter, but almost. Other points about the brushes:
You can see at least an approximation of what the brush stroke is going to look like as you are drawing. I don't like the path first, brush stroke when you're done method of operation, especial when I use the tablet.
(Big point for me) You can make and save your own brushes.
Other things that influenced my choice:
It works really well with the tablet - for most things I don't have to put the pen down and go to the keyboard.
The text objects don't use text boxes. I make posters for people sometimes and I found the textbox arrangement on Lineform very awkward.
The in place editing of gradients is pretty neat as well as being able to drag and drop gradient chips.
The snap to path, snap to point and snap to angle stuff makes some kind sof drawing much easier.
Lineform's pen tool works like every other pen tool I've ever tried. ZeusDraw's works differently - you enter the anchor points and wing points as you go along by clicking and you can go back and move any of them in the middle of inputing a path. At first I was a bit put-off but by this, but after using it a bit I'd never go back to the other way.
I like the way the viewing tools are done, especially having a "view undo" where you can go back to previous views.
As for some of the things that you mentioned:
the canvas seemed infinite and without any page margins.
Yeah the canvas is infinite, but if you don't like it, you can turn on the printing page boundaries. It's under the view menu.
The Lineform brush tool works very smoothly, it beats the pants off Zeusdraw (and even Illustrator) in that department. It's actually pleasant to draw freehand strokes in this thing.
I'm puzzled by this. Did you actually take some time to really explore either program in your "quick spin"? Line form seems to work by drawing a polyline as you move the mouse or pen and then replacing it with a Bezier path. If you move the pointer rapidly you get an obvious, non-smooth polygon. ZeusDraw seems to work the same way - except that you have the choice of turning the smoothing on or off and choosing the amount of smoothing. Did you turn the smoothing on when you tried ZeusDraw? I couldn't see any difference in the responsiveness between the two programs.
you apparently can't do selecting incidentally from within e.g. the pen corners tool. Way too modal.
Not sure what you mean - the command key changes the tool back to selection arrow while it is held down and so does the rh mouse button. I have the button on my wacom set to be the rh mouse button for this.
Inkscape ? The last time I tried it, it was like most other open source desktop apps - too many cooks playing with the soup. Lots of features, many of which are odd or don't quite work. Plus X programs look really crude after Aqua.