When we bought our 2000 Legacy L, we also checked a Legacy Brighton, but it was manual and my wife, despite being competent with the clutch, preferred an auto for comfort and resale value... (accountants, pfff.)
If I remember correctly, not many differences, mostly power/comfort stuff and no roof rails.
The headlights are similar, with a single reflector for both low and high beams (unlike the GT of the same era).