Do both. An MJ18 with 500watt amp would be a nice budget sub for the house. I run 2 15" RL-P's off of 800watts total and it's PLENTY of bass. Enough to wreck stuff in my house and has knocked out my projector bulb once.
But if you're that worried about the cost of electricity then it might not be for you as that's the least of the expenses involved to set up something decent.
But if you're that worried about the cost of electricity then it might not be for you as that's the least of the expenses involved to set up something decent.