That depends on the Pokemon itself. For eg.- Volcarona and Mega Pinsir have arguably the worst typing in the whole of OU. But they more than make up for it with fantastic stats across the board, unlike their breathren Bug/Flying types.
For a Pokemon that is meant to outright sweep the opponent team, resistances and immunties are hardly of any use other than gaining a switch opportunity. For set up sweepers, a balanced offense and defense is required, so both STABs and resistances are equally important. For Pokemon with stally outlooks, immunties and resistances take a higher priority than STAB (after all, Chansey is never run with a normal-type attack).
Now, as for Mega Golurk, it depends on how you've decided to spread your stats across. I'm not very sure about this, but the most effective stat spreading would be in Attack, Defenses and maybe, maybe in Speed. Whatever the case though, Golurk is always gonna be slow. This means it will usually have to tank an attack before lashing back. Here's where immunities and resistances would be useful.
At the same time, it needs a strong attack to follow. So a good STAB wouldn't hurt either.
Conclusion: Mega Golurk at it's best would be a bulky attacker. Both, a good STAB and resistances would be important for it to function. But it's not compulsory for the Pokemon to have STAB to hit hard. Mega Golurk, as a layman would picture it as, would probably already have sky-high attack.
So in my opinion, stick with the original Ground/Ghost typing. It actually gives it a pretty decent coverage already, and if you plan on giving it Iron Fist then Hammer Arm would complete your coverage.
Hope I helped!