You've heard some good advice from these guys, but let me add a few comments.
#1 - most important thing - buy a captive bred and born animal from a reputable breeder, preferably someone who has several years of experience with the type that you want - when you decide what type of boa to buy, post a message in the forum asking who breeds them. You'll find that there are guys that specialize in exactly what you want, whether it be a Columbian Bci, a Surinam Bcc, an Insular (Island) Bci, one of the many Bcc morphs out there, or the various locality boas. These breeders are typically much more knowledgeable of the boas they sell and are always willing to give advice or answer questions. That's more than I can say about the staff at Petco or many of the online retailers.
#2 - Ask lots of questions before you buy: "Does the animal have any health issues?" - mites, respiratory problems, scars, etc. "Is it docile?" - does it hiss, strike, or act nervous?
#3 - Don't let the initial price be a big factor - Decide what you want, then do whatever is necessary to get that boa (unless it requires selling you car or house). These animals live a long time. If you have to settle for your second, third, or fourth choice, you'll always wish you had waited until you could have afforded your first choice.
Now, about the generalizations regarding the differences between the various Bcc and Bci:
Don't think of a Common Bci as a beginner SNAKE - think of it as a beginner BOA. If you've never kept a snake before, consider a corn snake, king snake, milk snake, etc. They are much easier to maintain and don't get nearly as big. However, as a first BOA, you probably couldn't make a safer choice.
If adult size is a primary issue, buy a male - they don't get as big as females. In general, Bcc do get larger than Bci, and some localities of Bci are smaller than others, but growth can safely be regulated by feeding. A boa fed two mice/rats every 7 days is going to grow quite a bit faster in those first couple years than one fed one mouse/rat every 10 days. A lazy PET snake in a cage that hardly moves doesn't need as many calories as a wild animal or one that a breeder is trying to grow to sexual maturity as fast as possible.
If temperament is a big issue, Common Bci are generally your best bet. However, there are always exceptions. Talk to your breeder and other boa owners. There are nasty Common Bci, and there are docile Bcc. Personally, I have owned two Common Bci and two Peruvian Bcc. None of them have ever hissed or struck at me, and my Peruvians are as calm as any Bci I have ever been around.
Looks are a personal issue. I happen to appreciate the natural beauty of a perfectly patterened and colored Bcc redtail more than anything else. If you put a Common boa next to a true redtail boa, there's no comparison. But that's just me. Others see one of the spectacular Bci morphs and their jaws drop in awe. And yet there are others that are fascinated by many of the more rare Bci locality boas. When it comes to looks, go with your gut instinct, not what other people think.
That's all for now. Good luck!
Tyler