A complementizer attaches to a clause and allows it to be used as an argument. A nominalizer takes a non-nominal and converts it to a nominal.
と is pretty interesting now that I think about it.
太郎は［ジョンが手紙を出したの］を知っている。, here 〜の is a complementizer I think, because it takes a clause and converts it to an argument, and is also a nominalizer, because it gives it nominal properties (I think... does it?).
The 〜の in 走るのは好きじゃない。 seems to only be a nominalizer.
(In that, 走る doesn't seem to be clause there -- it's not just that the subject is dropped, but that there actually isn't a subject)
〜と seems to always require a clause, and doesn't provide any nominal properties, so I'd be inclined to call it a complementizer only, but I'm not entirely sure.
ジョンが手紙を出したの in that sentence up there seems to semantically mean something like "the fact that John sent the letter" which does seem to support that it's nominal.
Is "nominal" a syntactic or semantic thing? I'm confused now.
This always seems to happen when I start thinking about things and try to apply linguistics terms.