I run 1/8 on my DCM rigs. The barrel works well with 62gr and even better with 68+ when the distance requires it.
In regards to the 1/8 and the 55gr the twist is too much and seems to destabilize the round; group will open up due to too much spin.
My DCM barrels are 20" barrels, so keep that in mind. Also, considering that your going with a 16" barrel, and I doubt that the difference is really going to matter unless your planning on running match ammo or floating the barrel.
The other deciding factor will be directly related to ammo. If your planning on using a 5.56 Nato chamber, your going to have a hard time finding a round that will fit into the mag, and still be in correct position/distance from bullet to lands(OAL). The standard Nato chamber just has too much jump from standard bullet to lands, and if you reload for this ideal relationship, the ammo is never going to fit into the mag.
On one of my practice rigs, I have had the 223 chamber throat enlonged up to allow the use of standard ammo, but this sacrificed the accuracy. Since the rifle spends it time in others hands, and I don't have to have reloads to run, the sacrifice worked out well.
I guess what it boils down to is what is the purpose of the rifle. If your not going to use it for a Three gun event, then your best bet will to just buy the barrel with a 1/9 twist and run the 55gr or lighter that will suit the barrel/your purpose the best.
Just food for thought.