my advice: write on if you don't grade on. My school's law review is almost all grade on. We have a writing competition, and I think many people just don't take advantage of that method of joining. There have been years that our law review could have picked more write-ons, but didn't take as many as planned because the pool of applicants was small. Had a few more people entered a quality product, they would have been invited to join. Usually the write on process will span a week or two of your time. The payoff makes that time worthwhile.
Law review can be a lot of work, and the hours spent with your Blue Book can be oppressive, but overall, I found the experience to be the very best thing I did in law school. Employers definetly look for law review on the resume, but more importantly, you will learn to research at a level far beyond what most students learn from their legal writing courses. You will be an expert at citation form, and your writing will improve dramatically.