If I had to choose between the two, I would take Dez Bryant (prefer to trade up for Okung). Here's why:
Both are good return men (CJ is a bit better), but a player's ability to return punts and kicks at an elite level only lasts for a few years. After that, players revert back to their primary position.
With Crabtree and Bryant on the edges, and Vernon Davis in the middle, defenses would be absolutely befuddled. If defenses try to shutdown the WRs with a cover 2, VD will eat them alive. If they key on VD, the WRs will shred them.
With Spiller, he can be effective out of the backfield for 10 carries a game or so and 3 - 5 catches out of the backfield...maybe a bit out of the slot. Really limited in his long-term impact on the offense.
If vote for Dez!