Be careful with mixed class awards, if one leg is in business class, you should theoretically get charged both ways in business class, meaning your 87000 award ticket will now cost 125000 avios, even if the return leg is in Y or W.
The only way would be to use upgrade vouchers, then your award is still premium economy round trip, and one leg is upgraded to business.
If not using a 2-4-1 or having a stopover, it is usually better to book one ways, although if you cancel the trip, you must pay the cancellation fee twice.