If you have RPUs available and you aren't saving them for a Hawaii flight, you probably want to use them until they're gone, then switch to mileage-supported upgrades.
An RPU is worth up to 30,000 miles and $500 (if used on IAD-HNL) and as little as 15,000 miles (if used on an E fare on a domestic trip).
I tend to value miles pretty highly because I can use them to buy last-minute travel for non-elite family members while getting pretty good availability and reasonable prices. I find it's all too easy to accidentally let an RPU expire and find it difficult to use an RPU for friends' travel — R availability has to line up, it can be annoying to get the cert redeposited in case of irrops-related rebookings, and there's an extra layer of hassle in getting someone else's PNR. So I tend to use RPUs on myself until the need runs out, then use them on friends, then grudingly let them expire.