TPO is thermoplastic. EPDM is thermoset. Both are classified as nonbituminous roofing membranes.

You will use EPDM if you're a cheap bastard :)

TPO has only been around for about 15 years or so, and has performance characteristics superior than of EPDM. It will also cost more money, but will come with longer warranties.

Ah ha! Thanks Rusty.

One of the major factors is color.  Due to LEED and cities countering urban heat island effect, high reflectance/low aldedo roofing material is required.  And white TPO is usually cheaper or at least very competitive with white EPDM.  Also, clients tend to have preferences based on their history and familiarity with a product. (Can their building manager go up and plug a hole?)

Rusty, Postal, or whoever-

Do the methods of application differ greatly between the two, or are both systems applied with a heat source to bond the material to the roof substrate? 

Neither is heat bonded to substrate. Your three options are: loosely laid and ballasted (allows for greater movement of substrates), adhered (most common installation for non-protected membranes) or mechanically fastened (best for steep roofs).

TPO seams are heat welded. EPDM seams are adhered. 

Thanks rusty!

also take note: typically manufacturers won't recommend EPDM for a restaurant.  something about how the animal fat and the membrane react to each other.  TPO is preferred in this instance.  TPO is higher quality with the welded seams and all, but EPDM works just fine as well.  I've worked with lots and lots and lots of EPDM roofs.  and if i'm doing a green roof, the membrane will almost certainly be EPDM, all day long.  but, as rusty notes, both are very similar.      

I suggest you read the preeminent source on the subject:

http://www.wbdg.org/design/env_roofing.php

This is a great peer-reviewed resource that many of us whackjob enclosure designers use for obtaining basic concepts.

bossman, I never heard of animal fat eating away EPDM's. Hilarious! Thanks.

Why would you use EPDM in green roofs over hot fluid applied rubberized asphalt such as the one by Hydrotech? Seems like an overkill. No?

Token, that does look like a great resource, although (right off the bat) calling both TPO's and EPDS's as single-ply is something the industry stopped doing a few years ago (too vague). The correct term is Elastomeric Membrane. 

You are correct- this particular summary entry was last revised in 2009. The core concepts behind how they operate and are installed are still applicable, though.

I am sure all of the references at the end most likely have updated nomenclature, if one wants to follow through.

I don't know much about hot mopped roofs.  The builders I've worked with, especially out west, just like EPDM.  

TPO is shit and so is EPDM. BUR or Mod Bit is the only way to go[period]

EPMD:  Those were the days.