Although I now design mostly with Corel Graphics Suite (which is a great choice, by the way, if you decide to go with proprietary), I have some experience using GIMP and Inkscape for this kind of thing.
Most of your required tasks suggest Inkscape much more than GIMP to me.
- Navigation Tabs
- Glass-Looking Navigation Tabs
- Glass-Looking Buttons
Say I want to draw a tab with rounded corners. This is easy to do in Inkscape and is programmatically controlled using the properties of rectangles or the Node tool. What’s better, I can easily change my mind about the corners later and adjust them to suit me, or even go back and square them.
Same thing for adding a “glass” look to buttons or tabs; I can easily create derivative shapes with Inkscape and tweak gradients to my heart’s content. Last I knew, there was no easy way in GIMP to, say, adjust the midpoint of a gradient with a Node-type tool.
- Image Slices for Gradient Backgrounds
- Image Slices for 3-Column Layout Designs
Same deal with “tweakability” of gradients, but this is one task that might be almost as easy in GIMP as Inkscape.
- Icons (e.g. Shopping Cart)
I find “drawing” stuff way easier in Inkscape than GIMP - see the above benefits of the Node tool. In addition, complex layouts in GIMP virtually force you to proliferate layers - one per distinct object - and if you change your mind and want to enlarge something, you have to redraw it as there’s no scalability.
Absolutely GIMP, - no question. Many vector editors include substantial support for editing bitmaps, but Inkscape isn’t one of them.
Probably still GIMP, if I understand correctly what you mean.
I am not saying all these tasks can’t be accomplished in GIMP - they can. It’s just far easier to use Inkscape in most cases, IMHO.
As far as laying out complete web pages, I wouldn’t even attempt it in GIMP. Inkscape is pretty handy for that job, especially since they announced a fix for bitmap embedding in the latest version.
One weakness of Inkscape, while I’m thinking about it: Last I knew, you could only export to PNG format. This means that you will need to use GIMP, or another image converter, to encode your sliced n diced images to jpeg or whatever format you desire.
Just my $.02USD. Hope this helps.