I have a Pioneer SX-205, it's a decent enough receiver. I'd say it's only worth paying $75 for if it's complete with remote and is very good condition. It may be rated at 100 watts per channel, but is more than likely closer to 50 watts per channel.
I can't recommend the SX-550 unless it's prestine and as B3Nut said, it ought to be serviced at that price.
What avenues have you found these two receivers? In my opinion you should wait, which allows you to save up some extra money as well as find a better deal.