I think the present tense conveys a sense of immediacy (duh!) and power, but it can be hard to maintain immediacy and urgency over the course of longer works. It also seems to me to be more natural when used with a "first person" narrator. Also, it requires a great deal of skill on the part of the author to keep it from becoming trite. At least, that's my opinion.
The past tense, on the other hand, seems much more traditional and comfortable (which is not really a bad thing).
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