I have kept black pines for almost 4 years now. Just got my first pair of black milks last month, but have kept Andeans for about a year and a half (not the same, but similar in size/temperament/feeding response), so thought I could give you the pros and cons:
Black Pines: Pros- very curious, active. You'll see them out a lot...good for a display animal. Sometimes hissy when you reach for them, but generally calm once in your hands. Amazing feeding response...eat right out of your hand/tongs. Never musk.
Cons- need warmer than normal heat requirements for digestion (84* or so). Never seem to just stay still when handled. A big one keeps you busy holding it. Most are not solid black, if that's what you're looking for.
Black Milks/Andeans: Pros- laid back, lap snakes...the line I have (from Dennis Mountain) never musk, never vibrate the tail...calmest snakes I've ever held. No need for additional heat. These guys can digest food easily at 70*. Good feeding response, but shy (leave the rat pup in the cage vs. eating out of your hand). The Black Milks I've seen get blacker than the Black Pines and MBK's I've seen.
Cons- Stay hidden most of the time. That being said, I have an Andean in a display cage and he comes more often as he has grown larger. I imagine the black milks become more visible as they grow also.
You can't go wrong with either. If I had to choose one, I would go with the Black Milk...just because of the temperature requirements.
Michael

