In November, I arrived at Oase about 4pm on a Wednesday afternoon and found about 30 ladies with at least 10 in the top category. And not many other guys so the selection was with out much competition. The numbers of both increased in the evening - I left about 10. No complaints about the numbers or the quality during my time there. In fact, 4-6 was great - relatively quiet, easy to get drink, plenty of seats available.
Hope that helps.

Both have similar characteristics - more ladies in the evening.  That doesn't mean that the selection is any better, because there are often more men and fewer available rooms too.

If your time is limited to just an afternoon, I would recommend Palace, because it is closer to Frankfurt and it will therefore give you more time in the clutches, um, embrace of a pretty woman or two.

Palace can be a little more sharky than Oase, but if you are comfortable fending off wannas, then you will be ok.  Don't pick the first woman that approaches.

Thanks, both these answers were really helpful. What about weekday afternoons? Any days better than others?

I was at Oase during the day on a Monday and there were very few guys there until around 4 or 5 p.m. and plenty of women to choose from.  One thing to remember is that both clubs have a "lingerie" day (I forget exactly what it's called) where the women wear lingerie, rather than walking around completely naked.  I think it's Thursdays for Oase; not sure when Palace does it, but it's probably on their website.  So you might choose to avoid or attend the clubs on that particular day, depending on your preference.