Walk around and look at scenes you would like to photograph. If they usually look good with your eyes then buy a normal focal length like a 150mm. If you feel you would usually want things a little bit wider then buy a 135mm. The difference between a 135 and a 150 isn't great and if you buy one focal length and feel you need the other then just use your feet to get closer or further away.

As dirt cheap as 210mm lenses are I'd pick up both a 150 and 210. They along with your 90 would make a nice evenly spaced lens kit!