I love my 50-200 SWD. It's a nice fast lens with beautiful bokeh and quick AF. For birds I add a EC-20 to get it out to 400mm. Sigma also makes a 70-200 f/2.8 which would be another alternative, but I'd take HQ Oly glass over Sigma anyday.
The Oly 70-300 is also a very decent lens, but if you can pony up the cash you'll be MUCH better off with the 50-200. You can often find a used non-SWD (slower focusing, but still very good optics) on eBay somewhere around $650-700.
Mirror lenses are tempting, but they have some real downsides. If you're not careful you can get very ugly bokeh. They tend to have slow f/ ratios. The central obstruction in a mirror lens throws off center & spot metering, as well as making use of a split prism focusing screen tough. Also, depending on what you're shooting, acquiring your subject at 300mm+ with a prime could be very tough. I usually keep the lens around 300mm, find the subject, zoom to 400mm and focus, then shoot.
What's most important is to have fun with whatever you do pick up! In birding patience and persistance is key. Enjoy!