MoonLight0

I've been using whitening products recently (toothpastes and whitening rinse), and though I have seen a difference in my teeth, they're still not quite as white as I'd like them to be.

So I decided to try the Crest White Strips. Before buying them I decided to wait till they went on sale for 30% off and I also got a $5 coupon for the Premium strips.
But just yesterday I found out that one of the local stores is having a buy one get one free sale. So I decided to wait till today to get the strips only to find out that the ones on sale are actually the Classic strips, not Premium.
I also noticed that the Classic ones are cheaper and take up to 2 weeks for full results, as opposed to one week with the Premium.
So I thought about it and decided that the Classics are a much better deal. I'm not in a rush to whiten my teeth anyway, I just want them whiter eventually.

So I'm basically wondering whether I made the right decision. I've heard that these strips cause sensitivity (for most people), and I definitely don't want to experience that. So I figure whitening them slower for two weeks won't cause as much sensitivity.

Oh, and I also just did an online search and have read reviews from few people saying that you're not supposed to whiten the top and bottom teeth at the same time... So instead of whitening them in 14 days, it would actually take 28 days?
Is that true?
I've read all the instructions that came with the box and it doesn't say anywhere that you can't whiten both sets of teeth at the same time. It does indicate that it may be uncomfortable to wear both strips at the same time, so it's better to use them separately.
I'm kinda confused on that.