Hi Michael - Allow me to direct you to the following link. http://www.aerohive.com/products/access-points/ - this shows Aerohive's range of wireless products (which you may or may not be familiar with). Once you find the device you are looking for, click on the photo and the select 'view datasheet' this will give you all the information you need regarding radios and coverage maps.
As a general rule, all things being equal, the signal propagation of external antennas exceed that of the equivalent internal antennas.
The other major advantage of external antennas is that you can change the antennas if the signal propagation requirements change. With internal antennas you are "stuck" with the same signal propagation pattern.