Technically neither -
it's a CPU:
Complimentary Premier Upgrades are available on most United- and United Express-operated flights in the following regions:
- Continental U.S.
- Hawaii
- Alaska
- Canada
- Mexico
- Central America
- Caribbean
- Oceania
Last I checked DR is in the Caribbean! You can of course apply a RPU but it's a waste IMHO. Chances are you're upgrade will clear (mine did from BQN to EWR as a lowly Gold last year). I'd recommend looking at upgrade options and seeing how many F seats are available. If the answer is high (i.e. 9) then I'd hold off until either it drops to a low number (i.e. under 5) then you can use the RPU or better yet award miles (since award miles are less valuable than RPUs on this route since there's no co-pay) to confirm the upgrade.
You should really value your RPUs - those suckers can be used to upgrade flights to HNL from anywhere in North America and premium service routes like EWR to SFO.
Hope that provides some clarification.
-James