This is a bit of a ‘close to the heart’ kind of subject to me at the moment, because I’ve just had to swap from one marque to the other. Although an FH is involved in my case.
In answer to the question…Actros.
Without a second thought, moment of hesitation or whatever.
The FM doesn’t come close.
I have just been dragged (almost literally) out of my 55 plate Actros 2546 LS and put into a brand new 08 plate FH440. 
The Merc, despite having approaching 600,000 km on it, never missed a beat, whereas my Volvo broke down twice (loose earth ■■■■■■■■ the chassis) before it was ten days old - both incidents requiring a call-out from Volvo Action Service. The strut brace that is fitted ahead of the 5th wheel was also not fitted correctly. The securing bolts had no thread inside them and as a result just slipped off when you pulled them. 
The Volvo drops power to the 12V socket when you turn the key off which means the food in your coolbox goes off if you have a night at home. The Merc has a switch on the dash that you can leave on to power the socket.
The Volvo has no armrests on the driver’s seat as standard, whereas the Actros did.
The Volvo has no top bunk to sleep on as standard so I can stow my coolbox on the bottom one behind the passenger seat leaving the passenger seat and footwell free to use. This was not a problem in the Actros as it came with two bunks.
The speakers in any basic Volvo are crap, whereas the ones in my Merc were fine.
I miss the standard interior night lights in my Actros. They used to give a gentle green glow around the inside of the cab - just enough to allow me to find my bottle of water / CD collection. In the standard Volvo you don’t get this.
I also have to say that either bunk in my Actros was far more comfortable than the rubbish one you get in a Volvo.
And, I have to say, I the Volvo feels about 50 bhp worse off than the Merc. It just doesn’t pull like the Merc used to. And how I miss the growl from the old V6.
In short, for my job, I would have a basic Actros LS over a standard FH Globetrotter. As for an FM Globetrotter - I’d rather walk, thanks.