Yes, you nail it, keeping the Bitcoin network secure is very expensive and energy-intensive. Keeping NXT/NEM network secure is neither expensive nor energy-intensive.

Since Bitcoin was released in early 2009 it has set the standard in the cryptocurrency field. Until today its Proof of Work mining system is the most popular and frequently used model of assigning value of digital currencies. With Bitcoin's staggering rise in popularity over the last five years its network has grown to such massive size that it has become impossible for the average user to mine and large-scale, energy-intensive mining operations have taken over, GHash.io now having accumulated 51% of all hashing power. Its energy demand and resulting real-world environmental costs lead to the critical question of whether this extremely energy-hungry system is sustainable -- while more secure and energy efficient technologies are available. In the energy-hungry world of today, Proof of Work is not a smart and sustainable choice.