Parallel doubles the mah while Series doubles the voltage.
For parallel, Using 2x3000mah batt will give you 6000mah with equal discharge on use, better for lower watts vaping.
For series, giving double voltage will give you more power on wattage which means better for 50watts and up.
In terms of battery life, that depends on your vaping style. Just base it on the above.
By the way, used both Therion 75 and 166. 75w work well on 15-30w vaping, but won't last long when vaping around 50-60w, you'll feel battery snug too. 166w gives better batt life with high watts vaping.