Are these your first brick-films?
I've never done any brick-films myself, but both had a lot of jitter. I know it's hard to make sure the iphone is in the same location everytime you take a photo if you're holding the iphone in your hand and putting it down everytime to move the minifigures.
I would suggest proping the iphone against something and not moving the phone unless necessary.
In the end, though, I thought both were pretty good, especially if they're your first brick-films and for using a free app.
Personally, I think the Capt. America one is shot better and the story seemed to flow better.
EDIT: I thought I'd seen iphone tripods, and found Mini Adjustable Tripod+camera Holder for Iphone and Other Cellphone
on Amazon currently selling for $4.60.
There's also a tripod adapter, Stand with Mini Tripod Mount Adapter for iPhone 4G (Black)
for $7.39 on Amazon.
Not trying to pressure you into buying anything, but thought it may be worth the investment if you plan to do more brick-films in the future. There are some negative notes on some of the user uploaded photos, but they're rated at 3 and 4 stars, respectively.
Can you use a standalone digital camera to take photos, copy them to your phone and use the app to make a movie, or do you have to use the app to take the photos?