I couldn't decide whether to buy Collonil Waterstop spray or Collonil Leather Gel, so I bought both and I emailed Collonil to ask what the difference was between them. They told me that they both offer the same level of water protection, but the spray has UV protection which the Gel doesn't have. So the spray will stop the sun fading the colour of the leather but the gel won't. They both stop the dye coming off leather onto light coloured clothes too.
The spray has a very strong chemical smell, so you have to cover your nose while you spray it on, but the smell goes once it's dried, and it dries quickly. The Gel doesn't have this strong chemical smell, but I think I will be using the spray more than the gel as the spray has UV protection whereas the Gel doesn't.