With a big, long-established tour company, you may pay a little more, but the higher cost should mean a high quality experience. Smaller companies often can provide more personal attention. If a smaller, newer company is offering a tour for substantially less than its competitors, it's wise to delve a little deeper to make sure that you are getting what you pay for. Talk to members of your local bird club and anyone else who is a frequent traveler. Chances are good that you know someone who has taken guided tours.
Personal recommendations can be very helpful.
If you've never gone on a tour, start with a relatively short excursion in North America to see how you like the experience, rather than leap into a three-week marathon trip to some exotic country. You should also consider the pace and intensity of the tour. Some trips are relaxed and others are gung-ho. Start off with an easy trip and plan for a more challenging one later. The tour company can describe the pace of the trip for you.