With so many transcription/caption service providers out there, you need to carefully choose a service provider so that your valuable video is treated professionally; not just a quick turnaround time with a low price. Below are some guidelines:

1. Understanding the purpose of your video 

Do you need captions for more viewers globally?
Do you need captions for better search results?
Does your video include special lingo and terminologies?
Does your video include heavy accents, grammatical errors, slang, etc.? 
Does your video have mixed use of languages (e.g. English and Spanish)?

2. Understanding of various video technologies and formats

Does your video need blank spots for TV broadcasting (terrestrial, cable, and satellite)? 
Is your video intended for Internet and/or mobile smartphone use? 
For Internet use, will your video be compatible with flash standards used by YouTube?
For mobile use, will your video be compatible with standards used by Android-based cell phones or iPhone?

3. Protection of Intellectual Property Rights and Copyrights

Do you want your vendor outsource your video to a third party/country for cheap processing?
Who will have their fingers on your valuable videos?
Do these people have non-disclosure agreement signed?

4. Secured Data Storage

How long will your video be kept? And who can see them?
Will the video be downgraded to save disk storage requirement?

5. Quick Turnaround and Reasonable Price

Will the video be sent overseas for low labor cost?
Will it be easy to make any changes on the video, if you have to?