The only differences I've found are:
With the Signature AP:
Different site to access photos
Unable to add borders/stickers (I was told they may be adding this in the future)
Only download is an option (can't order the cd)
No print packages at character meals (but you get a redemption code to add the pictures to your AP, these didn't work and I had to send the code to customer service to add them.
Longer access to photos (a full year)
A selling point for you may be that DL has suspended the pre-sale option, so it is now only available in the parks for $100. If you just get one signature AP in your family, it is $250 more, so if your are planning on 3 or more trips, it pays for itself right there. Depending on your spending habits, you could break even with just 2 trips (making up the extra $50 in the extra discounts.