Assuming you're planning on staying for the long haul, I'd look at how long it would take for you to recoup the higher initial investment for higher efficiency/tankless water heaters. CR had a article about this in 2008 and according to them, both tankless and high efficiency storage models took 15-20 years before they recouped the initial investment through annual savings. The Federal Credit will definitely offset that time period, but you'd need more efficient (i.e., more expensive) models to qualify for it, at least that was the case when we replaced our furnace.
Here's a tidbit from CR in Oct '08 in regards to maintenance issues.