First of all, as the old story goes - the questioner should learn neither, he should learn Mussar. After learning Mussar properly, he/she will realize that there is time for both b'iyun and b'kius learning.
Based on the Bavli, Megilla 6b
ואמר ר׳ יצחק אם יאמר לך אדם יגעתי ולא מצאתי אל תאמן לא יגעתי ומצאתי אל תאמן יגעתי ומצאתי תאמן הני מילי בדברי תורה אבל במשא ומתן סייעתא הוא מן שמיא ולדברי תורה לא אמרן אלא לחדודי אבל לאוקמי גירסא סייעתא מן שמיא היא
Rav Yitzchak said, "If a person tells you, 'I have tried and not succeeded.' Don't believe him. 'I have not tried and I have succeeded.' Don't believe him. 'I have tried and succeeded.' Believe him. These statements are regarding Torah study, but regarding business he is helped from heaven. And regarding Torah learning this statement can only be said about learning which sharpens a person, but regarding memorization he is helped from heaven.
With this in mind, it would seem that one should engage in b'iyun learning as opposed to b'kius if one has to choose, because the results of b'iyun as a direct results of ones efforts, whereas the results of b'kius are not. Theoretically if one learn b'iyun his whole life he could both be sharpened and memorize everything, whereas if he only learned b'kius he would certainly never be sharpened and may not even memorize everything!
Inspired by this question.
