Ignore him and get the 50.

1. The B.K. has a high bottom bracket which makes that seat tube length deceiving. The standover height will be probably be an issue on the 52 and certainly on the 54 frame.
2. The frame also has a steep seat tube angle. You'll probably have your seat back a bit further on the rails than a slacker road bike, etc. (assuming you want to maintain the same position over the cranks) and that will effectively lengthen the top tube.