Both forks will be worthless. There will be no winning fork.
That's also a very likely possibility, but only if everything splits into 2.
If 90% people go with the fork, the ones left behind will lose though.
people will dump their bitcoin before the fork occurs because nobody will want to take the risk
If you buy up Bitcoin before the fork, then don't move it (keeping it in cold-storage (ie: off-exchange)), you get to "bet" on both forks at once.
The risk comes when you take up
mircea_popescu on his "
buy gavincoin at 25% off" offer. Edit: no takers so far.
Presumably, if you need to spend Bitcoin while waiting on the sidelines, You can buy some at an exchange, then spend immediately: assuming both your exchanger and retailer are using the same fork.
Edit: tvbcof has been bringing up an interesting question: how will SPV wallets react to all this. I think it is worth investigation. I suspect SPV clients will try to look at the block the transaction is included in, which means size is important. If a forking block is buried and not examined, the SPV client will probably side with the fork with the most hash-power.
Incidentally, that is how consensus is achieved in Bitcoin: the fork with the most hash-power wins.