Hi Thor5517,
It's a good question, and a complex one. Part of my answer has to be vague, drawing on experience: whether you'll get a rapid response or a slow response from Columbia seems to be a matter of luck. If there's a pattern there, I've never been able to discern it, and I've been working with Columbia's rolling decision for ten years now. I've even seen the occasional unlucky ED applicant pushed into RD, a sort of waitlisting, and as I recall, in April of the following year the poor applicant was still waiting for an answer. So there is no set timeframe for outcomes in this case, at least not as far as I can see.

To understand the difference between ED and RD, think about it from Columbia's perspective: why would they have an Early Decision period in the first place? Why is Columbia's SO early -- earlier than any other school? The answer lies in the issue of the "Yield Statistic": how can Columbia protect its yield numbers from being damaged by applicants they like but who are also liked by, say, Wharton and HBS? ED does that. Columbia fills as much of its class as possible with people who are locked in, i.e. who will absolutely accept their offer. So there are major advantages for you as an applicant in shooting for Early Decision.

If you go for Regular Decision, you'll be balancing Columbia's yield concerns (if he likes us so much, why didn't he apply ED?) with being the first one in the queue in a rolling-decision system. There is no reliable science for all of this, and lots of conflicting opinions floating around out here.

So it's a complex series of questions, but ultimately, the answer is really very simple: if they like you, and if you seem committed, they'll give you an offer. If Columbia is your first choice, definitely do ED. If it's not, definitely don't.

Hope this helps,
Margaret Strother