Differences SPA501G/508G? Headsets?

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What would be the differences between the SPA501G and the SPA508G? They're both apparently 8-line phones. The pics looks pretty close.

Also, are there any good headsets for these phones (do they both take the same headset) which don't require bulky external amplifiers? Wired and wireless suggestions would be welcomed.

 

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Woops, I now see the 501G doesn't have a display, the 508G does. Answered my own questions there. Still interested in headset information, though.