Depending on when they were made, the only difference will possibly be chipset, heatsink design and die size, though all those factors can be the same if they were made in the same month or something.
Well consoles are locked at 30FPS and are stretched on higher HDMI resolutions making them look worse.
There should be no performance difference between the consoles as they use exactly the same hardware, excluding the chipset.
The best place is probably Amazon if they still sell them, ebay is probably the best source for old Xbox 360s but be careful on there.
And if you ever hear 'arcades are crappier than the pro and elite because they always break or whatever' then that is BS, the arcade is the same as the pro and elite minus the chrome disc tray cover, the hard drive and the black plastics of the elite and some other accessories - just as a side note......