I'm not a fan of tankless heaters, but aside from that, you will almost certainly need to increase the power service regardless of which you go with. Neither gas nor electric services are large enough to supply enough power. As far as cost is concerned after the initial outlay, gas is always cheaper that electricity. In most applications, gas is faster too. I use gas for everything possible in my home and even though our electric rate are about the most reasonable anywhere in the US, gas is still less expensive.