The value of BTC after the fork will presumably be the value of BTC + BCC after the fork. If BCC has any value at over the coming months, then that is effectively yours but inaccessible. And if it retains value, Coinbase is going to be in a very awkward situation where they're sitting on an asset that may or may not be yours any more.
-- EDIT:
Actually, the savviest move is probably to sell. Coinbase's position mean that it's more than likely that a lot of BCC is inaccessible even if it has value. That means BTC drops with no way to recover the lost value. Either the value of BCC drops to zero (and BTC recovers over time) or Coinbase eventually releases BCC (people who held it get BCC equivalent to the loss in BTC value). Either way if you sell immediately before the fork and purchase when it drops, you should retain the lost BCC value.
(This assumes that BTC itself doesn't lose out).