I chose other local driving instructor, this is just based on my experience. I have used a local instructor, found her to be a lot cheaper than my 2 friends lessons who went with BSM, even when they paid for bulk lessons, I also found they were told to have a lot more lessons than was maybe necessary etc. but really its your choice. My instructor works weekends a lot of the time too, which ive found useful what with college etc. she was quite flexible, although sometimes she did cancell/ swap lessons but she also didnt charge if i couldnt make a lesson at last minute. which i doubt big companies do.
Even if your nervous theres nothing to say the AA will be any better than a local private instructor, as both have dual controls and will be used to nervous drivers. Talk to some friends who have learnt/ are learnign near you and see what they recommend etc.
hope you enjoy your lessons.