Personal preference, but opposite of what kelv likes, I HATE mismatched tint. I think the whole dark back windows and lighter front windows like an SUV is ugly. Just a pet peeve of mine.
My Mini has 20% tint all the way around. It looks great and it isn't hard to see out of. One thing to keep in mind, have it done right by a professional with high quality tint. Your ability to see out of it well, and for it to last long without bubbling or discoloring is the quality of the product and work, not just the percentage.