There are all sorts of advantages to teaching your child ASL that especially apply during the first two years of life. These advantages include an average (spoken) vocabularly larger than that of non-signing peers of the same age as well as an even larger vocabulary in sign than their non-signing peers have, earlier and clearer communication development (on average) and even earlier spoken communication (again on average).
The only one of these advantages related at all to emotional well-being, is that they can communicate sooner which means fewer temper tantrums before they can verbally communicate fully.
Research is showing that most advantages to baby sign go away after the child is between two and three years of age - even the signifcant increases in early vocabulary for most kids listed in the first paragraph. The other kids simply catch up sometime between the ages of two and three. However, I still recommend it because it was enriching for all of us during the infant and toddler stages. My daughter was able to tell us her favorite animal was an elephant before she was 16 months!
If you use ASL instead of baby sign and keep it up even after your child is speaking fully, there are a few further advantages that are lasting. For starters, your child has the potential of becoming fluent in ASL and being bi-lingual in this way (a huge andvantage no matter how you slice it - read "The Bilingual Edge" for more details about advantages of multiple languages for kids) but you have to be pretty comitted to it to make this work. If you live in an area where there is a deaf community your child may have access to friends in the community it would be hard for him/her to converse with otherwise - admitedly, there is a smaller liklihood this advantage applies, but worth considering if it does.
To use ASL instead of baby sign. Use videos from the "My Baby Can Sign" and "Signing Times" series instead of other signing videos. You can also use ASLpro.com as a wonderful translating resource.
I'm so sorry I can no longer site sources. I did a lot of reading on the matter about six years ago and though I read multiple studies to back up the information in paragraphs 1-3, I just don't remember what I read. Most of my sources were print sources as well (making it hard to include links), but those facts do come from genuine research and not sales sites. I hope this is helpful anyway.