Wren seems performance oriented. Tau is security oriented but will also be very high performing. Tau is more powerful but Wren is something which can be implemented immediately. Sometimes to expand on your momentum you can implement something immediately even though you might swap it out for something better later.

Steem (Graphene) is the highest performance blockchain architecture available. Steem can achieve mass adoption because it can scale and be fast, and Wren would allow Steem to leverage it's design advantages. Tau on the other hand is much more powerful but is not finished.