I guess with the Falcons I personally and I think a large portion of people didn't expect much from them. No doubt they were a good team, but I was kind of scratching my head wondering how the Falcons ended up being the team representing the NFC in the Superbowl. So as soon as things started to go down hill for the Falcons and the Pats started to gain their footing I had a feeling the game would make a turn for the worst, especially since the Pats always seem to comeback into games in this fashion.
As for the Hawks they were pretty solid favorites in that game, one could argue maybe not favorites but at the very least a 50/50 equal to the pats. The Hawks staying in the game and making a crazy comeback when things seemed to be over was just fantastic. Then that all got taken away by one fatal call.
I think it hurts more when you can look back and pinpoint exactly what you did wrong and know it was that single dumb mistake that cost you the game, rather then looking back and thinking hmmmm there were a lot of mistakes at the end and it all just fell apart.
I think it's worth noting that expectations are not confined solely to the lead up before games; they can (and will) change as a game progresses. With that in mind, I think it's pretty fair to say that while quite a few people didn't think Atlanta would stand much of a chance before the game, there were very few people who didn't expect them to win the Super Bowl when they took a 28-3 lead. Heck, if you had offered me a $100 bet on the Super Bowl winner as of 10:00 left in the 3rd quarter, I would've picked Atlanta without a second thought. Atlanta's loss is worse, in my opinion, because just about everybody was damn sure the Falcons were going to win at multiple points in time. In XLIX, there was still sizeable doubt until the end. Lynch had been stood up on a 3rd and goal from the two yard line earlier in the game, so it wasn't a guaranteed score.
As for being able to look back on one mistake vs. a whole bunch, I suspect that depends a lot on how exactly you define a "worse" loss. However, I suspect it'll be a much more demoralizing loss for the Falcons than the Seahawks. Seattle was in position to win and made a stupid mistake; that's one thing. You know where you screwed up, and you know how to fix your mistake if you get in that position again (at least, theoretically, Seattle apparently still tried passing from the one yard line in a regular season game). If you're the Falcons, and you just blew a 25 point lead, where do you even begin? There's so many different places where things went wrong, and you know Atlanta is going to have a doubt in the back of their minds whenever they get a big lead next season. The psychological impacts of collapsing like that are enormous, whereas a single stupid mistake doesn't get into your head in quite the same way.