Tankless Water Heater (and Hot Tub Conversion?)

We are looking into replacing out electric water heater with a tankless gas model and it occurs to me that there is no reason we shouldn't be able to also have it serve the electric hot tub also with a heat exchanger of course to keep the spa water out of the domestic supply. We keep the hot tub hot all the time which is normal for the electric "portable" spas. The guy at the hot tub company says "no way" but I don't see why. He said the constant low flow demand is just inefficient with gas but it still doesn't make sense to me. The tankless on-demand heaters are suitable for radiant heating and other low flow tasks. Just need a thermostat to give little 30 second boosts to the tub as needed. Maybe there is an inefficiency with starting that up and running the water through the long distance of pipe from the house to the garden (about 50 feet).
Also, for the tankless water heaters, does anyone have advice for the brand and model to use? We are just two people with pretty low requirements. The stubby short electric heater now never runs out, it's just a matter of reducing monthly expenses. The Bosch Aquastar 125B NG is about $200 less expensive and doesn't require an electrical hookup but the flow rate is only 3.6 GPM at a 50 degree temperature rise (other models are twice that). We wouldn't mind turning the dishwasher off when showering but don't want a cold blast if someone does a bit of hand washing in the sink while showering. Also for the location, I'm thinking it would really be nice to have the new heater closer to where it's used. The current delay bugs me and probably wastes energy too.
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I'm getting a recommendation for the Noritz brand but don't know if the others are much different. We'd like to install it outside but the Bosch with no electric is only for indoor use.
Paul Furman wrote:

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