6 year 12 year?...the cost of the heater itself can vary by as much as 100% I have seen basic electrics in the 350 range with higher end gas at 700 or 800.
Installation costs are about the same. I recently replaced my 40 gal propane unit. With some added new valves, re-works, Stainless flex lines etc I got out the door for a bit under 800. I then spent as few hours re-working my supply and feeds line to allow for a simple flex line connection in the future. ( I relocated my meter too)
A year ago or so I replaced an "indirect" heater for a client. This Triangle Tube Stainless steel "lifetime" tank cost $2200. An indirect heater uses a external heat source to heat the tanks water via a coil heat exchanger.
Have you considered in line gas water heaters, they are quite efficient now and save a lot of space. Another plus is not running out of hot water.
Look into local government gas distributor sponsored programs, you can get tax rebates/incentives or grants that can really help with the cost. My sister was able to get a tankless water heater installed for free (programs covered materials plus labor to install it).
Those are large heaters, If you have two in line to provide enough water for large tubs etc, you should consider a tankless system that will provide the same amount of water when needed without wasting any energy. Cost of higher end units run in our area around $3,500. That includes short side wall vent, gas pipe within 10 feet and simple water connections.
Unless you have an extremely large family one 50 gallon heater (gas) should do the trick. Look up reduced flow shower heads and faucets. If your a bath person use less water and share time with your mate. I am an electrician.