It's an easy comparison, you generally don't even need to look up the specs. The Giant is lower cost and has better components than the Trek. This is true in just about every case when comparing Giant to Trek (or Specialized, or most any of the other brands).

But if you want to get specific:

Trek DS3 - $879.99
Shifters - Acera
FD - Acera
RD - Alivio
Cassette - HG20 (Tourney/Altus)
Crank - FSA Dynadrive (low end square taper BB)
Brakes - Shimano M315

Giant Roam 1 - $820
Shifters - Deore
FD - Deore
RD - Deore
Cassette HG50 (Deore)
Crank - Shimano T521 (Deore Treking w/Octalink Hollowtech BB)
Brakes - Shimano M365

Like I said, for $60 LESS, with the Giant you get an awesome bike with high end components that will last you a lifetime. With the Trek, you are paying more for pretty much entry level parts across the board.

Also, in case you didn't know - Giant manufactures Trek's frames for them (and others - Giant has the largest frame manufacturing facility in the world). That's one of the reason's Trek is more expensive, they are buying the main part of the bike from their competitor and slapping their own logo on it.

Cheers
TRJB

PS. Full disclosure - I ride a Giant Roam 1. It's a fantastic bike, I love it. Like some of the others, I shopped all of the brands when looking. It's not that I'm a Trek hater, but they are just way over-priced bikes across the board, and that counts for a lot in my book.