Well, I flew in Y but with a Silver card (i.e.: lounge access) last year, and that's the best way to go about it. Obviously, if you haven't got that option, the SLOW lounges are exceptional (halfway between LHR Galleries and LGW Terraces, IMHO) but the quality of the Y product on-board is so much better than the European equivalent that it's questionable whether that aspect is worth upgrading from.

So, in summary, it's all about the lounges!