This depends on many factors that you'll have to think alot about. First, do you plan on applying to allopathic residencies only, osteopathic residencies only, or both (lots of cons about picking this one IMHO, but that's a whole other issue). If you plan on applying to osteopathic residencies only then of course you only need the COMLEX. If you plan on applying to allopathic only or both, in my opinion you should take COMLEX & USMLE both Step 1 & Step 2. Now some will say... but allopathic residencies will accept the COMLEX, it's equivalent, blah, blah, blah... but in the "real" medical world there are still ALOT of people (higher up's that will be looking at your residency app) that do not know much about the COMLEX and do not want to put out the energy to figure out how it compares to the USMLE (which will be what most of the applicants will have taken if you're applying to allopathic residencies). In my opinion... why even risk that you may get your app into the hands of one of those people... why give anyone a reason not to consider you. Now, I don't think it's right that DO's are still dealing with this but it's reality, and we have to just deal with it. It doesn't take that much more effort to take both (it sucks, but it's doable), it's just a bit more money spent.
Oh, the thing about taking only COMLEX Step 1 & then both COMLEX & USMLE Step 2 is also a particular point that you will hear many opinions about. Again, only my opinion... just take both for both steps. Some programs look at Step 1 more and some look at Step 2 more (for those who have taken it by application time) so why take the chance. Just have them both for both steps and they'll have NO questions or concerns.
OK, hope that helped a bit. Gotta get back to it!
Any more questions.. let me know
~Bean