Hello,
Personally, I'd go with a black milk, I have a pair that are about 18 months old (see attached pic of female), and I think they're one of the best snakes out there. Here are some things to consider:
Black milks start out as tricolored babies, and gradually darken up during their first couple years of life. They are very easy to maintain, don't need any sort of special heating or light, and don't require any sort of special permits to keep or buy across state lines. They also won't hurt you if they bite you... although both snakes have very docile temperaments, they are also both very aggressive feeders, and if you accidentally make the sort of stupid feeding errors that we all do from time to time (nobody's perfect!) the black milk will never send you to the ER for stitches. Also, if you're interested in breeding, black milks are very easy to breed, they don't even require brumation like a typical colubrid. Pretty much the easiest snake in the world to keep.
I've never kept an indgo, unfortunately it is illegal here in Maine. Check your state laws; depending on where you live, it may not be legal for you either. Unless you can find a breeder in your state, you'll have to get a federal permit... not difficult, but a bit of a hassle. Indigos are more expensive, both initially and in terms of feeding, cleaning, and housing. They have high metabolisms and love to eat, then spraypaint the walls of their enclosures with feces. Black milks leave nice neat little poops on the substrate that are easy to scoop out with a small plastic cup. Indigo cages have to be hosed off and wiped down frequently. Since indigos are not constrictors, they have much more powerful jaws than a black milk, like a pair of pliers made of razor blades. Get your fingers between them and lunch, and you'll be in a world of pain. They will also need a much larger enclosure. Breeding indigos isn't easy either, even the most experienced keepers have seasons where, for no apparent reason, they just can't get a successful clutch.
I'm not trying to turn you off to indigos, they are one of the most beautiful, powerful, intelligent, and majestic species out there. If it were legal, and time and money were no object, I'd have a pair myself. Indigos are the reason I got into black milks, actually. But, indigos require a lot of work, dedication, and patience. If you just want a big black snake that looks cool and is easy to take care of, the black milk is definitely the way to go.
Just my opinion.
Nate
