It is really nice to have a constant temperature, endless amount of hot water.
Since you are talking about electric, assuming you are installing the tankless in the same location as the tank. That would minimize the electrical costs. Also safe to ignore all the comments about how much longer it takes to get hot water with a tankless. There is no difference in output, just a difference in plumbing pipe distance.
There is a yearly flush that is beneficial to removing the scale that builds up. No anode rod to replace.
Size is measured by GPM and temperature rise.
An alternative I wish was done here is to have several smaller tankless heaters. One for master bath, one for 2nd bath, one for kitchen, and maybe one for laundry if not near one of the others. More cost, but near zero lag in getting hot water.
Operational costs should be less overall. You need the same amount of electrical energy to heat the water - maybe less because of efficiency. Need zero energy to keep a tank heated.
You will gain some space.
Check prices at big box stores, and at plumbing supply houses.
Replace the expansion tank too. Put a valve on it to make it easy to change in the future.
Many manufacturers offer a dedicated valve manifold. Get it. Makes installation so much easier, as well as performing maintenance.
While running power, try to have an outlet put in. Handy for a leak detection device. For either type replacement.
If you are hiring a plumber, check forums for recommendations. Also for people that have way overpaid for a F "happy" plumbing place.
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