Are the pdf's mainly text? If that is the case the sony readers will allow you to reflow the text to fit the screen. The kindle 3 does not do that. I have a sony 350 and a kindle 3, and pdf is far better on the sony. That being said, if it is just a txt pdf, I can convert it to mobi using mobipocket creator and it looks great on the kindle. The problem is, I have a lot of pdfs, hence having two readers.
If there are a lot of charts and graphs in your pdf files, go with something larger like the Kindle DX or the Sony larger version.