"In the end, I would make the decision based on the estimated risk the pipe becoming energized." Chris, There is legitimate discussion about the need to bond the hot water to the cold at the water heater since until recently it was connected by other means and it isn't specifically called for in the NEC. Your statement that "They probably already are bonded through a shower valve in the house" leaves open the possibility that they aren't - which is the whole point here. Bonding is not the same thing as using the piping for a grounding electrode. The NEC is very clear that all piping must be bonded to the electrical system. It's not an option to decide if you want to bond the piping to the electrical system. Most certainly you don't get make a decision that "you don't think it will become energized, therefore you aren't going to bond it" as suggested by your last sentence. Think of the implications if an un-bonded water pipe comes in contact with an electrical wire and then you go to take a shower, or do dishes, or any number of other things. If it's bonded to the electrical system, it causes a breaker to trip. If not, everything it touches is energized.

You state that you live in Philadelphia. I bet you didn't know that Philly was the first city in the USA to have a plumbing code. Ben Franklin was an avid bather, bathing sometimes as often as once a week; which was unheard of in his time (imagine that). In addition to inventing stoves and writing fire codes, Franklin also wrote the first municipal sewer code in the mid 1700's. As a result, Philadelphia and New York City have some of the strictest code enforcement departments in the world. As a result of that strict code enforcement and a high cost of labor the prices in your area will be slightly higher than the national averages. We don't know how long the gas line will have to be or how long the vent has to be so it is difficult to even venture a guess as to what your final cost should be, however, those prices do not seem to far off the published estimate prices. Now having said that, there is another option that you perhaps have not explored. Let us assume for the moment that you were to select the 80 Gallon Electric @ $1,095. Further assuming an estimated 12yr life span that would then break down to $91.25/yr or $7.60/mo. The 75 Gallon Gas Direct Vent @ $2,525 over 12 yr would equal $210/yr or $17.53 a month. Now keep in mind that you have to put the total cost up front to get those installed, but most Electric companies have a lease program. We got an 80gal commercial water heater, fully installed and the old water heater hauled away for a monthly lease fee of $14.95 billed on our electric bill, and they are responsible for all maintenance during the entire lease period. The only additional cost we put forth was $21 for the permit @ inspection, which we would have had to pay regardless of who installed the water heater and a couple pots of coffee that I shared with the guys while they were installing the heater. Now I am a retired Master Plumber and I could have easily installed a new water heater, but when Jan told me they would supply the heater, do the installation and be totally responsible for all future maintenance for a mere $15 a month I jumped on it.

Have to disagree with GreenDesigns who states, "they offer almost zero improvement in efficiency of a new high quality tanked electric heater. ...electrical tankless is only truly practical if designed for a new build from the beginning. Retrofitting is extremely expensive and won't give you a payback for the cost differences inside of your lifetime." I put a whole-house electric tankless in my 1906 house and will never store water in a tank again. Absolutely love it! It's mounted on the wall, directly to the side and just above the dryer. I did not bother to hide the water lines inside the wall, but since they're [mostly] hidden by the dryer, who cares? Sometimes, putting plumbing pipes in the wall is highly overrated. Mine is in the bath/laundry room and I've never found it to be unsightly. Yes, the tankless requires quite a bit of power: Mine requires two, double-pole 60amp breakers with 6g wire. So, here's where distance from the electric panel is a cost consideration.

"Get an adapter for 1/2"..far more common.." It can take enough torque to remove a badly seized element to damage a 1/2 inch drive. I am always amused watching folks try to remove a tight item by pushing down on the breaker bar. You can only exert a force equal to your weight that way. Pull up on the bar. You can generate more force than you weigh using your muscles. I have encountered a few elements so tight the water heater itself started to move. before you drain the heater try to at least move the element. The weight of the water is to your advantage here. As soon as the element breaks loose, THEN drain the heater and finish removing it.