Neither is really correct (German speaker, here).
Lees is the name for yeast at the bottom of fermentation in French. Trub is the same thing in German. We generally use lees when talking about wine and trub when talking about beer. Technically, they should both be pronounces in the same way as in their mother tongue.
In the case of trub, both "Tr-uh-b" and "tr-oo-b" are mispronunciations, assuming you're pronouncing the R in the manner that Americans tend to. Troob would be the closest, but in German the R is pronounced more like a G (similar to the way the french pronounce an R).
That said, both troob and trub are terms used by homebrewers. You might get somebody who's a stickler for either pronounciation, but at this point, both are incorrect in German... and thus, there isn't really an argument for EITHER. People will understand you either way.
Also, don't be discouraged. Pronouncing a word based on how it looks is upsetting to people. But the reality is, if you're learning how to pronounce based on spelling... you're making it known that you're a reader. I'd rather work with somebody who's done a significant amount of reading than somebody who knows how to needlessly pronounce words in the technical manner.