Hi & welcome!
A few responses... None are really bath-safe as is, but I think that you could get third-party sealable bags that would suit either!
I'd expect that the Cybook or the Sony would both do well - see
here for a explicit comparison on the wiki but it probably comes down to how important a particular difference is to you.
The Sony's available at £199 & while the Cybook used to be ~£249, WH Smith recently had it at £149 but it's unclear if that's a loss-leader, mis-price or what. Note that the Sony comes with a decent case, whereas the Cybook doesn't, so the difference isn't as clear-cut as it may initially seem.