I am a DO student, and have always had the attitude that it made sense to take USMLE if you think you are going to score higher than the average of people who are applying in the field you are looking at. For example, I believe the average USMLE score for ppl who match to psych is low 220s. If you think you will score higher than that it makes sense to take them. In a more generalized sense, it makes sense to take the USMLE is you think you are going to score higher than the overall mean. Otherwise, why take an exam that shows you are below the average? Especially considering that since you are even thinking about taking the USMLE, you probably scored pretty well on COMLEX. Why would you want one test score showing you to be, say in the top 1/3 percentile of ppl who took it, and another one showing your in the bottom 1/3. To me it would make sense to just have a COMLEX score in that case. Of course you can't really know how you are going to score on either, but you can use the resources you have to give you at least a reasonable approximation of how you compare to other students (ie, if your in the 90% on Uworld you aren't likely to bomb your actual test).
I would also like to add that you shouldn't be scared of USMLE. Although I did not take USMLE step 1, I took step 2 and did very well. I have not gotten my COMLEX step 2 score back, but I can't imagine it being too much better than my USMLE score, and percentile wise my USMLE step 2 score is significantly higher than my COMLEX 1 score.
I guess what I'm saying is that if you are a person that generally tests well, odds are you will do well on USMLE so you might as well take it. If you are a person who just barely sneaks by on tests, or hovers right around the mean on COMLEX it doesn't make all that much sense to take USMLE.