I've owned both, but not at the same time. I loved my GMT, but I always found the crown dug into the back of my wrist. The watch had a reassuring heft that matched its utilitarian looks. As much as I loved it, I moved the watch on as I found it just a tad too large.
When the baby DN was announced I just had to have it. It was every bit as well made as the GMT and felt just a premium, however the smaller size made it much more wearable.
Now, I have sold the baby DN. It was ultimately usurped by the Everest Jubilee which I bought as a lovely birth year watch for my son. Were it not for trying to keep to a max 3 watch collection, the baby would have definitely stayed.
Given the choice now, I'd pick the baby DN.