What has a lower long term cost per hash: LTC, BTC or other alt coin?
One of scrypt design goals was to make CPU and GPU performance balanced, which meant that it was far more dependent on memory speed. 

The GPU had a solid work around and as far as I know CPUs don't have any advantage.

But that did cause it basically impossible to do scrypt mining with an ASIC unless it has a ton of memory and memory channels added in, which is a huge design problem.  Unlikely unless GPU mining for scrypt is very profitable.

This also makes Scrypt far more energy intensive for the same amount of useful hashing, but the network adapts as it requires far less hashes to solve problems (Difficulty number)

Which is greener? By far ASIC mining, it's power usage is tiny compared to performance, 4500MH for 4.5w vs 275MH for 100w.

But again, the network balances.