There's a few games missing from both those series, actually.
But both are great and well worth getting. Especially Wing Commander, IMO. In fact, I'm considering picking up the GOG version of WC4 even though I've got a physical copy, just because it probably runs better and it's easier to set up the DVD quality movies.
EDIT: May as well go into specifics.
Ultima is is missing the two Worlds of Ultima games, spinoffs based on the U6 engine that send the Avatar into different settings inspired by Victorian literature. Savage Empire is a pastiche of explorers in tribal lands, while Martian Dreams is sci-fi (and features Jules Verne as a party member). It's also missing Ultima IX, but nobody cares because it's awful.
Wing Commander is missing Wing Commander Prophecy (the fifth 'main' game), Academy (a simulator simulator), Armada (a sort of 4X with active battles), and all the expansions (WC1+2 should have two each, Privateer should have one). And Secret Ops, which was a standalone expansion/sequel to Prophecy, but is freeware anyway.
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