I guess it depends on which one you're more interested in? My personal bias is towards OOAD but that's just me. I can't really back that up with anything else but a gut feeling that knowledge on OOAD would be more valuable in the long run.
RUP and OOAD are two very different. RUP (Rational Unified Process) is a SW project management methodology where ITERATIVE development is enforced. The basics of it says that the product prototype should be made ready very early in the development phase and one should build on the top of it. OOAD is a SW design/development methodology in contrast to the Structured design way. By doing research on these, if you mean the aim is to find out if these methodologies are useful or not, then I think RUP may be a good topic because this is pretty new and not much of such research has been caried out. I personally feel this is a fad, a way for the project managers to force the developers to develop the product quickly. Otherwise, doing work on OOAD is simpler because this is an already established and proven process. Thanks
RUP prescribes a lot of activities and UML artifacts for OOAD, so they're nicely related. If you're interested in processes, a survey of current trends would be a possible topic. I always recommend Alistair Cockburn's Agile Software Development for a survey of numerous agile methods. His website has a lot of good papers to read, too. Here's his old site: http://members.aol.com/acockburn/ which has links to the new site plus a few things that haven't moved yet.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi