How much did you pay for Zoom, is it becoming more affordable? I used to use GO-SMILE and it worked great before, it's more expensive than the whitening strips.
I've been in dentistry for...well, a long time and honestly, I would not do an in-office whitening procedure UNLESS they are having a great special going on. Typically, offices will charge $500 or so here (Seattle), but some dentists will run specials for half off. Even that is still expensive, considering the products in the stores do work, just not as quickly. The in-office whitening is basically a boost/jump start, then you have to use trays with whitening gel for a while until the desired shade is reached. Then you touch up by using whitening trays every so often. In-office whitening can cause sensitivity as well and that's a major issue for many people.
Lots of dentists offer Groupon specials on Zoom. I've seen it as low as $99. I've never tried it. I'm perfectly happy with my Pro-White. Only available on-line. The Pro-White site doesn't sell good trays though. I got my trays on Amazon. Well, I got the kit to make the molds/impressions for the trays. It came with an addressed & postage-paid package to use to send in to the lab, where they use your molds to make your trays. Then they send the finished trays to you. All included in the initial price of the kit. I think I paid $59 or $69. Most dentists charge $300'ish to make similar custom-fit trays.
I also have used the crest white 14 day package. I didn't use them everyday though because it made my teeth too sensitive. I used them once a week.
Sometimes the sensitivity is caused by too much of the gel getting on your gums. Try to make sure the teeth are covered but the gel doesn't get pushed up over your gum.
I use the crest white strips with advanced seal, they stay on even if you drink water, no slipping. Also, baking soda with a squeeze of lemon works too!