I think both books (or series, as it where) are excellent. I've never read anything by Pratchett or Adams that I have not enjoyed tremendously.
But I would have to say I think better of Hitchhiker's though, as well as Adams himself... Sorry Valkary. They are both awesome, but that's my preference.
An "alternative" to Hitchhiker's is RED DWARF. In case you have seen the TV show of it, rest asured it is NOT a Hitchhiker's ripoff. I must admit I thought so, when I saw the TV show, I immediately thought it was. But when I read the books, I found they were completely unique, and different from Hitchhiker's -- it's just the TV version of it that seems to immitate it.
As far as Good Omens go, my fave religious/anti-religious book is SCEPTICISM, INC. This is one of the only two books I have ever bought more than one copy of, so that I could lend it out to more people. It is one of the most enjoyable books I've ever read. If you cloned Douglas Adams, Terry Pratchett and Kurt Vonnegut Jr., you would get the author of SCEPTICISM, INC. I'm not kidding: The story is told by a conscious, intelligent shopping trolley, on his way to the top of Mount Everest, to resolve a spiritual crisis.

I would consider it a great loss in my life, had I never read this book, it's that great, and funny..