There are plenty of ways to whiten your teeth, but there are two major competitors in the market. In results, both products are fairly competitive. In a week, they both offer visibly whiter teeth. So which should you choose?

Crest Whitestrips

Crest Whitestrips come in various strengths and styles, but the basic model is the same for all of their products. Crest Whitestrips Premium stick directly to the teeth, holding the gel firmly to the teeth. Depending on which variety you use, you may have to leave the strips on anywhere between 5 minutes to an hour per day (half an hour in the morning and half an hour at night).
Crest Whitestrips aren't particularly uncomfortable to wear compared to whitening trays from the dentist, but having plastic strips stuck to your teeth for half an hour isn't incredibly pleasant either. It may cause excessive saliva, especially during application (which is a problem, since the strips won't stick to incredibly wet teeth).
Crest Whitestrips may also cause sensitivity in the teeth for a few days, but it shouldn't last.

Aquafresh White Trays

Aquafresh White Trays are newer on the market, but have now been around for a couple of years.
The trays are effective, using a 10% hydrogen peroxide concentration in a plastic tray filled with gel. Some people complain that the gel can get clumpy and whiten unevenly, but with proper application, they should still whiten effectively. These trays may actually cover more area than Crest Whitestrips. However, the trays may not fit all mouths equally and are less comfortable than Whitestrips. Even if they fit, they are more difficult to keep on than the strips, but unlike the strips, they only have to be worn for half an hour per day instead of for an hour total like the Premium 7 day formula.
A major consideration with the Aquafresh trays is in the effect on your gums. Unlike Crest's product, which is fairly easy to put up against the gum, White Trays are harder to keep only on the teeth and can cause the gums to be bleached and swollen. This can be prevented by putting Vaseline on the gums before applying and by wiping off any extra gel with a dry cloth. It's also a good idea to avoid brushing your teeth before using the product, as brushing can cause tiny abrasions in the gums and will absorb more of the hydrogen peroxide.
Finally, it's more difficult to find inexpensive White Trays. Crest Whitestrips are usually about $10 cheaper, possibly because they whiten more gradually. If you need something fast and only have half an hour a day, Aquafresh may be your product, but it will probably be less comfortable and a little more expensive.

In either case, the best thing to do is to follow the directions. The people who have problems - who get white spots or severe pain or other complications - are the ones who try to use a product meant to work gradually over a week or 10 days in one day. Both products will give you brighter, whiter teeth if you can let the product work as it's meant to work.