I would not ever recommend an external scaler with a projector of the class of the AE900. Panasonic tends to have very good processing built into their gear, and the AE900 is no different. Good scaling.
But, the 2600, if I remember correctly, uses Faroudja deinterlacing, which is also highly respected for quality.
Which is better? I can't say for sure, but the obvious advantage of the scaler being built into the receiver is that you then only need to use the single HDMI output from the receiver to the projector and you can leave your projector on JUST the HDMI input to achieve the best quality image. Plus, the 2600 is simply a great receiver on its own.
In reality though - neither will do standard satellite a lot of good. Standard satellite is good, but it isn't even close to DVD, and is that much further behind HDTV in quality. Upconverting 480i SDTV leaves you with 480i SDTV... that has been converted. It doesn't ever make it anything close to what HD looks like, or even what DVD typically looks like.
My opinion? (since you didn't ask) Get a cheaper A/V receiver and get a BETTER satellite receiver with HD capabilities! This will cost about the same, but will deliver to you the absolute best image you can shoot for. With the AE900, I would call it the ideal way to do things.