Crest Whitestrips every time. I get them on ebay and they last forever. You don't need to use all the strips, you get quite a lot in a box. Might feel funny the first time you use them. You will notice a difference almost straight away and then you can store the rest until you need them. I've had a box now for almost a year and have used about half. they don't seem to harm the teeth but they make your fingers sting when you pick them off the backing strip !
Which? has recently reported on home tooth-whitening and strongly advises against it. To be effective the hydrogen peroxide concentration has to be very high and this can severely burn tissue inside the mouth.
Having it done professionally is completely different- and rather pricey!