First Bike

The Giant has the slighly better groupset - I've heard the Tiagra shifts a little better than the Sora. First hand I wouldn't know. But being a yank, and a yank in NYC where most folks seem to ride a Specialized or a Giant, the Boardman has a higher "wow" factor (on this side of the pond anyway). Maybe they don't even sell them over here, who knows. I get a whole lot more compliments on my bottom of the barrel Aluminum Cinelli with Ultegra than I ever did with my Dura Ace equipped Tarmac SL3. That's certainly no objective criteria for a bike purchase but every compliment tickles me pink, and maybe Boardmans are a dime a dozen in your area. Either bike however seems like a very nice first bike as long as it fits you right.

Most major bike manufacturers will have a model in this price range with similar components. Most folks would be hard pressed to articulate the differences of bikes in these price ranges. Take a test ride if you can. Personally I would buy the bike that speaks to me (in my case it would be the Boardman for the reasons above), knowing that I would probably end up upgrading many of the components on either bike down the road.

Edit: Let us know what you decide and post some pics. Welcome to the club!

If you can get a test ride on each bike, do it and make your choice based on which one you like best. Tiagra components are slightly better quality than Sora, but just barely. Most riders wouldn't be able to tell the difference in shift quality between the two, especially when they are both new. But when coosing any new bike, base your choice on the one that you like best and the one that fits best. Since a poorly fitted bike is usually uncomfortable, the one that you like best and the one that fits best are normally one in the same.

Thanks for the advice guys, I'll get test riding! (just wish the frost would go away)