My Zuiko 85mm f/2.0 is tied for being my favourite lens (with my 35mm f/2.0). I don't own a 100mm f/2.8.

You cannot go wrong with the 85mm f/2.0. I expect you cannot go wrong with a 100mm f/2.8 either.

What other telephoto options to you have available to you? If you have either a zoom that covers a similar focal length, or a telephoto (such as a 135mm lens) which is somewhat close, one of the two focal lengths may make more sense.

Do you often find yourself shooting wide open with your other lenses? If so, the extra stop with the 85mm f/2.0 may be very important to you.

If you are looking for maximum portability, the 85mm f/2.0 plus a 2X tele-converter gives you a nice, small 170mm f/4.0 as well.