Stealthfixr - I am by no means an expert, but do have a little experience in this area. Firstly give consideration to what lenses you are wishing to use and determine whether they fit into the EWA bag - remembering to take into account any zoom or focussing extensions in the maximum measurements.
As with anything you get what you pay for, but it is a waste of money purchasing something which won't satisfy your needs. Give consideration to how much fine tuning you need to do in the process of your snorkel. If you want to have the ability to access functions - find a rigid housing. If you want to set everything and then just click away - with difficulty pressing the shutter half way through a thick plastic barrier - go the EWA bag.
If you want to dive i'd personally give the EWA bag a miss, but some divers say it's ok. I have been told that there is a greater chance of the bag flooding whilst snorkeling than diving, because there isn't enough pressure to seal the strip where the screwed plates press together. Apparantly a lot of 'accidents' happen when trying out in the pool and that's when a 'proper' housing is decided upon.
Also when doing your sums, look at the ports which are required for each of the rigid body underwater housings.
With the advancement of Digital SLR's, you may be able to pick up a second hand rigid housing.
Also check into your camera insurance in case of flooding. None for EWA bag. Some companies will cover for rigid housings.
Hope that helps a bit.
A good forum to visit to get heaps of info for underwater photography is ;http://www.wetpixel.comhttp://www.wetpixel.com/forums/