What is a better gearing combination for downhill? One of the tracks I've been riding recently is testing both my skill level and my bike. The track is a combination steepness and roughness that makes it significantly different to what I've previously riden. The track is probably more suited to an AM FSR but I'm riding an XC hardtail.

The frequent chain slapping and rear derailleur bouncing around has led me to question what the best gear ratio to ride is and if this will damage my drive train. 

Is it better to ride downhill in the larger chainring (42T) on a lower gear with a tighter chain or in the middle (32T) in a higher gear assuming the ration at the end is roughly similar?
I ride a hard tail over all sorts of terrain. Generally I ride on the middle chain ring in these situations. Its less to do with the mechanics, and more to do with whats possibly coming up, and fast + reliable gear selection. When on the large chain ring, you can get caught out by a short (or long), steep uphill, and not be able to get a low enough gear to climb it just changing the rear cluster. 
I reserve the large one for easy predicable ground, or tracks I know really well - for instance sealed/gravel roads, manufactured fast tracks with sweeping / bermed corners where speed is all important, and you are unlikely / have no need to slow down below about 20km/h.

If the track is steep and rough, I am unlikely to run out of cadence before 'balls' - if you find you (like many others) have a "lower center of gravity" than me (i.e. smaller brain and/or bigger balls), and you run out of cadence, then go for the top chain ring. Also the chain can be dropped far more easily off the top chain ring - although on a well tuned bike this should not happen. 

As far as chain slap and derailleur bounce - I ignore it. Its a mountain bike designed to be used and abused. If you want, you can get neoprene coves for the chain stay which will quieten it down, or wrap it in duck tape... I have never heard of derailleur damage caused by slap, but that is not to say it does not happen.