Companies are comprised of people. If you break your word and cost the company money, you're costing those people (and the investors) money.
As a business owner, I find it very frustrating how some people's morals seem to not apply to their dealings with corporations. I wonder if it's a lack of familiarity with how businesses are run, and the ease of treating the company as a faceless entity.
You may let down a few people at the first company if you change your mind and take another offer, but if you're a manager at a company then you should be well versed in dealing with the mild disappointment arising from an individual choosing to act in their own interest rather than the interest of the company.
Acting in your own interest is fine. Acting in your own interest contrary to your word is not.
If working for the company is not in your interest, don't accept the offer. It's as simple as that -- nobody's feelings will be hurt. If you need more time, say so. If the company won't offer you more time, don't accept the offer.
However, if you accept the offer and then renege, the people involved are likely going to have a validly poor opinion of your professionalism, ethics, and maturity.