It's a completely different window. Shares only the glass. The 500 has the slimmer frame, but does lose some structural integrity to the 800.

The 800 is a little stronger with a slightly better air infiltration number. I like the look of it a tad better. The 500 is still solid.

Another vote for the 800 from me as well. I like the strength of that unit and the metal handle/hardware is a nice look.

Do you know about how much bigger the frame is on the 800 series?

about 1/4" per side, 1/2" overall width.