Difficult question and one that only you can really decide, as it very much depends on your personal preferences.
I wouldn't want to be cramped up in economy so for me I would choose PE both ways,
but having upper class to look forward to would make a nice end to the holiday and if you feel you can "survive" economy for the outbound flight, then the prospect of returning UC would be much nicer than PE! I've always said that, despite UC being extremely expensive, it offers better
value than PE - the service and onboard accommodation is exceptional.