I realise this is an old question but I thought I'd answer it anyway since this is a decision I had to make recently.
They have very similar stats, but machamp's are better.
Machamp also has less weaknesses but golurk has immunities.
Machamp also gets STAB on its D-punch.
Personally I went for machamp because he deals more damage. The low speed is a problem for both of them but you could maybe give it a focus sash so that it doesnt get 1 shotted and then hit it back hard.
At the moment I use quick claw on mine which is risky since it's not always going to work, but when it does its essentially a free kill so I don't mind.
Not sure if its a good idea but you could always baton pass some agility to it. seems like a waste to me but it would make machamp pretty hard to deal with.
Moveset I would suggest is:
D-punch
Stone edge
Earthquake
Ice Punch
Ice punch is mostly to deal with double weak dragons (dragonite salamence garchomp) since stone edge does more damage to birds and a neutral D-punch still does more damage than a SE ice punch.
There's nothing there to deal with psychics because most of them will out speed you and 1 shot you anyway so you're better to just switch out. If you want something to deal with psychics you could always swap ice punch for payback and hang on with that focus sash I mentioned earlier.
Hope this isn't too late to be helpful.
