I've been to Oase once but just didn't like the Club, the layout is all very bitty.
Unfortunately although I went in August I picked a bad weather day so no girls were outside which was a shame as the garden area looked good, very large area.
However Oase does have a Kino which Sharks unfortunately does not even though it's an "open plan" Kino.
Personally I prefer Sharks as there are always plenty of girls to choose from and having been several times I generally know how to spot the good & bad service providers.
The bar area is very large with plenty of space and seating to observe the action and pick your prey. The garden is smaller than Oase but still a decent size, I love to visit in the summer.
It's very easy to get there from Frankfurt central or the airport.
The negatives for Sharks is it can get very smokey, no Kino and the food imo is poor.