The difference: Instead of each player having 5 frogs, there are 9 frogs that BOTH players are looking for. So your opponent will see your shots - will use the information you got in their next move (and you will use theirs).
So you have to be careful to not give your opponent too much information - and at the same time, don't be afraid to take more guesses since if you wait to figure out 100% where the frog is, your opponent will just swoop in and take it away from you!
coan.net: Unfortunally, it also means that it's an advantage to shoot squares where you will already know how it will reveal 0 (like shooting in the corner if your opponent shot (diagonally) one step away from the corner revealing a 0). Such a shot reveals no information at all.
AbigailII: Yea, it will be interesting to see how much cat/mouse type of play - that is would it be better to just make a dummy shot that you know will show a 0, or take a guess when you have a 33% chance (or 50% chance) - since a wrong guess will only cost you 3 points.
It will be interesting to see if a "safe" game or an aggressive guessing game will work the best.