In a situation like this, I'd stick with the devil you know. British Airways Dreamliner is a very poor experience in Y, for example.
If it's about the comfort and not just wanting to try it, just wait for the reports and use those to your advantage for future travel. For now, just go with the convenient timings. That's a certainty vs. the uncertainty of a slightly more comfortable ride on 787. Besides: it's still gonna be Y. The difference is unlikely to be stellar.