I would recommend that you speak with a plumbing company for the benefits of each one. This is usually not something that will add money to the bottom line when you resale the home, however if you are planning to be in the home for 3, 5, 7, 10 years and you will enjoy having hot water "on-demand" then it's probably worth it. Personally, I have 4 teenagers and if my water heater went out I would likely go tankless (just so that everyone could shower without running out of hot water!).
Coldwell Banker Jane Henry Realtors
North Texas Top Team Realtors
You mentioned your tank is in the attic and this is a contributing factor for the above average cost I'm sure. It is also the reason that my vote would be to go tankless. Water hearters are a funny thing. I've witnessed newer tanks fail and the water damage, while most likely covered by ins, is not worth the savings.