In general this will boil down to choosing max item level gear with sufficient Piety while maximizing Crit, then melding Crit > DH/DET where possible, though Piety melds can be useful in a pinch if you end up too low.
Piety is a binary stat, as any more than you needed to not run out of mp was wasted stats. Because of this, when choosing our substats, avoiding excess Piety is the best way for us to increase our damage, and you’ll sometimes see our low piety sets use lower iLvl equipment to avoid Piety (this is still the exception rather than the rule, don’t actively use lower iLvl items to dodge Piety without calculating out the gain yourself). It’s hard to predict exactly how much Piety you might need in most settings, but you can take a look at how much the recommended sets take in each setting and decide which one fits you best.
Remember that slower GCD tiers use significantly less mana (one less Glare per minute is 400MP saved per minute) and can thus get away with less Piety. If you find yourself struggling with MP, ensure that you’re effectively using your mp tools before adding more Piety. Swapping GCD heals for OGCD heals, using Assize and Lucid Dreaming on CD, using Recitation for a free GCD heals in a pinch: these are all effective ways to make your MP go farther. Also make sure to always carry HQ Super-Ethers when raiding, they are a very effective way to recover from multiple rezzes or a death.
Due to the WHM changes in 6.0, WHM now needs more piety than other healers to avoid running out of MP in optimized settings, so even BiS sets will take one or more piety slots.
Direct Hit vs Determination DET was buffed slightly in 6.0 which brings it close to DH but not quite to the point where it becomes a better damage stat for healers since we have 0 DH baseline. The healing boost from DET is unlikely to save a GCD heal or someone’s life, so BiS sets will mostly meld DH over DET where possible. That said, since the damage difference is so small now you won’t lose much melding DET over DH, especially for prog.
You can find stat tiers here: http://allaganstudies.akhmorning.com/stats/