Yeah, always beer. I figure that if I had the chance to swim in water or beer I would choose beer. Billions of yeast cells have thanked me for the beer bath.
To that point, I recall reading some years back that storing yeast under water (IIRC, distilled(?)) resulted in lower yeast viability in subsequent batches vs. storing the yeast under beer, usually the same beer it had just fermented.
Anecdotally, I've found that to be true the hard way; several years back, I had a couple of batches turn out less than stellar, and I was able to trace it back to using yeast stored under water vs. beer.
The slurries weren't that old (less than a month between harvest and re-pitch, and stored cold in my lager fridge), but the beers ended up "off".
Haven't made that mistake since.