Seems to me it was just barely over 10° a couple days last week in Eugene. How long do you stay out, and do you ride when temps are in the 30's and it's raining?
If you don't ride in the wet and don't stay out that long, you can probably get away just with leg warmers. If you're only going to get one pair of tights, a pair of medium weight ones with a little fleece on the inside is probably the ticket. If you ride in everything, neoprene tights are really handy when it's cold and wet or particularly cold. I'd get two pairs of tights at different weights if you can.