Premium economy is pretty much all about seat pitch and width. There may be added benefits (perhaps an extra suitcase but that may not be the case especially with a mixed class flight) so you need to compare the specifications of each airlines seat and see if you think the difference is worth the money TO YOU. No one else can make that decision on your behalf. If everyone thought it was worth it, no one would fly economy, and vice versa. Only you know what the extra cost and legroom means to you.
Having said that, i personally I wouldn't be interested in doing it half way, I'd either be paying to do the whole way in WT+ or find a company with a seat pitch in economy I could live with.
One word of warning, if you do upgrade, once you have done it, you are likely to be dissatisfied with economy for ever more.