To finish my contribution to this thread: I did a thorough test with the Image Sizer plug-in. This plug-in does give you the tags to work with the dimensions of the thumbnails and it generates them on the fly.
I now converted my portfolio from Gallery to weblog entries using this method. This gives me more options to mix images with the rest of my weblog content. If I choose I can now just use an image for a portfolio, but also a more traditional weblog entry with text and an image.
This could also have been done with the gallery, but it feels more secure using one method for everything. Especially mixing weblog categories with gallery categories and entries can get confusing.
The advantage of the gallery is that you have more control over your existing images. The image library of the EE entries is not so easy to maintain. Also image upload is much easier with the Gallery. But if you want full control over the way your content is displayed and mixed, using regular weblog entries is the way to go.