Grinders are well and good until meltas show up, 'tis true.
I wrote up on Daemon Princes on my blog
. Essentially, they boil down to a handful of good builds.
Flying Nurgle Prince. He's expensive, but very tough and extremely killy. Excellent pick, especially if you're maxing out on MCs.
Shooty Tzeentch Prince. Cheap BS5 bolt on a moving platform with a reasonable amount of durability and CC capability considering the cost.
Pavane Slaanesh Prince. Pavane can be used to replicate Fleet or help out other units, while your high I, grenades and Musk can help you kill enemies before they can swing back (important for T5 5++, Iron Hide is essential) and use H&R to cross the board quickly.
Cheapo Area Denial Prince. Dirt cheap walking face smasher, equivalent to Bloodcrushers.
Any other set up is either going to be too expensive, not survivable (T5 5++ really sucks for an expensive MC), just not focused (flying Tzeentch DP w/ bolt and might, walking Khorne DP kitted out with everything but wings) or are replicated by other units, only better (flying Tzeentch Prince w/ Breath < Flamers).
Efficiency is very important for Princes, as is focusing them to fit whatever gap you need them to fill.