Understand the temptation with Hasselbeck. He had a great first game. I'd consider going with Garrard though. Optimism is always high with Hasselbeck when he's playing, but he gets injured constantly, and defenses will be teeing off on him with Seattle's iffy running game.
With Cutler, you'll play him almost every game anyhow, so in a backup you need consistency and a guy who won't lose you your fantasy matchup. Garrard has been really consistent the last few years. He doesn't have gaudy TD numbers but has thrown for over 3500 yards the past two years and 15 TDs both years. Plus Garrard gets a few carries per game and consistently adds 25-30 rush yards per game and 2-3 rushing TDs each year. He will rarely get you more than 25 pts in a week, but he will also rarely get you less than 18.
And in terms of reliability, you're more likely to have Garrard there when you need him later in the year. Never know with Hasselbeck.