Maggy- as others have said there is a difference between safety sausages and SMB's, though they can be used interchangably.
Sausages have valves and are designed to be inflated on the surface and held high or waved to call attention to yourself. Think visual whistle.
SMB's generally have open bottoms, are designed to be inflated at depth, and floated up to mark your position before you ascend. They don't work well as wavable flags since since they lose air when lifted out of the water.
Ideally, you'll want to learn to deploy an SMB before surfacing, since as others have noted it adds to your safety, but this requires a greater amount of skill and good bouyancy control. As you inflate an SMB underwater, you'll be holding an object of increasing floatation which will tend to pull you up with it until you let go, and then there'll be the line which hopefully will run free and not entangle you. With practice this is actually easy to manage, but is a skill that has to be learned and used regularly to keep sharp.
If most of your diving will be with organized groups, and your primary concern is the offchance possiblity of separation, I suggest you keep it simple and buy a valved safety sausage and inflate it after surfacing. Consider that if you're separated from the group, you'll already be anxious and won't want the added stress of dealing with an unfamilar task and another set of problems.
blush
