The main differences ...
Baby Signing Time: Alex and Leah are cartoons
Signing Time: Alex and Leah are real
Baby Signing Time: all the kids signing are babies/toddlers
Signing Time: the signing kids vary in age (still includes some babies and toddlers)
Baby Signing Time: song pauses to teach a sign, next song segment is about that word, song pauses to teach another sign, and so on
Signing Time: a group of signs is demonstrated, then there's a song about those words
Although, from some descriptions I've read, Signing Time Series Two interrupts the songs to teach signs. I'm not totally sure about that since I haven't seen any for myself.
At 18m DD prefers "regular" Signing Time. She seems to like the age range of the kids, and she recognizes the "real" Alex and Leah. She doesn't accept that the cartoon versions are them.

She refuses to call them Alex and Leah! And purely for myself, the song interruptions drive me
crazy, so I prefer Signing Time too.
I would start buying Signing Time (making sure they're the Series 1 DVDs, not the Series 2), especially for a 15mo. They'll probably hold his interest a lot longer. On the website (signingtime.com) there's a PDF "progress" chart you can download to see all the signs that are on each DVD (I use it so I don't have to go through every single DVD description). I'd start with the one that seems most interesting/useful to you guys and go from there.
