I would go to the reader matrix (
https://wiki.mobileread.com/wiki/E-book_Reader_Matrix), look through all of the different features and see which ones are most important to you. Come back and ask questions if there's something in the matrix you don't understand.
The Sonys have better PDF reflow than other models. If your technical journal articles have lots of tables, charts, or diagrams, a larger screen reader, such as the Kindle DX or Sony 950 might be your best bet. I wouldn't get too hung up on which models work with which formats, as Calibre can convert to the format your device uses. Personally, I don't get too excited about WiFi or 3G because it's so easy to connect the reader to the computer and load them. For screen type, I'd go to someplace like Best Buy where you can see several of them together. The color readers do not use epaper screens. The ones that I've seen aren't as clear and easy to read as epaper.