Hello all! I own both gels...
I bought the classic gel for my Bayswater, as I understand it, it's cleaning as well as nourishing. So far, I have only sprayed the bag. I will use the gel, when I feel it needs it or when there might be a stain (hopefully not!).
At my parents' place, I have stored another spray and the leather gel. I bought it because I wanted to impregnate a new Tod's bag there - couldn't wait to use it! The classic gel wasn't available then. I think on the Tod's I only used the leather gel and did not spray - didn't want to overdo it. Or did I spray, too? Can't remember... And I have used the leather gel to nourish my tortue embossed Longchamp which worked well (the bag didn't need waterproofing but seemed a bit dry).
So: I think the classic gel is fine to clean and nourish but you should use the spray to waterproof. The leather gel is useful to waterproof and nourishes a bit. Anyway, I try not to use these bags on rainy days! But in daily situations like handwashing I can easily wipe away the drops which might have gotten onto the bag.
Hope my experience helps!