For raw efficiency, tankless units are pretty good, but suffer efficiency losses for small volume-draws (eg. a 2 quart draw for hand washing.) Standalone gas/propane tank HW heaters have atrocious standby losses, an only make sense if you're using way more thag 60 gallons/day (which you won't if you have solar as your primary heat source.)
See: http://www.aceee.org/conf/08whforum/presentations/1a_davis.pdf
If you're doing solar, using a tankless as your backup-heat to maintain a minimum temperature guarantees that short-draws never occur (it heats up many gallons several degrees at a time.) This keeps the total efficiency of the tankless very near it's steady-state thermal performance.
The often-used alternative configuration is it use a tankless in series with the solar tank. In this case the burner never turns on unless it needs to (saving standby losses on the tank, which can be allowed to run a lower temperature), but it loses significant efficiency in flue-purges at every burn cycle. In most cases that turns out to be a bigger loss than the standby losses of a solar tank (since solar tanks are VERY well insulated.)
In light to moderate heat loads commonly found in FL, using the tankless (with or without solar) can make an efficient combi heat/hot-water system, either using a hydronic-heating coil in your air-conditioning's air handler, or using baseboards/radiant-floor/radiators, etc.. Or if you don't have ducted AC, there are small tankless-heating-specific air-handlers available, eg: http://www.rinnai.us/hydronic-air-handlers/. Some example systems tested:
http://www.toolbase.org/pdf/fieldevaluations/Tankless_Hot_Water_Heater_EvaluationSWD.pdf
http://dsp-psd.pwgsc.gc.ca/collection_2007/cmhc-schl/nh18-22/NH18-22-106-108E.pdf
Takagi and Navien in particular are promoting their tankless units for space heating applications.
Check with local codes to see if ASME-tested units are required in these applications. ASME testing is usually for only the high-output commercial units in most tankless vendor's product lines, which is WAY more tankless than you're likely to ever need. Your hot-water load represents your peak burn-rate demand, your coldest-hours-of-the-year heat load is likely to be on the order of -15-35% of your hot-water peak, so it doesn't take a monster sized burner to deliver both (especially if it's buffered with a solar tank.) If ASME is a requirement, a smallest-of-the-line condensing-modulation heating boiler may be less expensive than a commercial tankless HW heater, but more expensive than a standard-efficiency tankless.
Depending on the installation, if the tankless is to be installed indoors it's sometimes cheaper to go with a higher-efficiency condensing tankless + plastic exhaust vent vs. a standard-efficiency tankless + stainless-steel exhuaust. The performance difference isn't as large as their steady-state thermal efficiency or EF numbers might suggest (especially as backup for a solar), but if the installed price is the same, you might as well take the few percent real-world improvement.
In FL, as just a hot-water heating system (but probably not space heating) it may be cheaper to go solar with electric backup in the form of electric-element in the solar tank, since propane prices are more volatile than electricity. It may be cheaper to operate, but may have a higher carbon-footprint due to the larger fraction of coal-fired generation in FL. But it will be certainly cheaper up-front to go with solar+ electric than solar + tankless. The other solar/electric solution to go with a tankless-electric in series with a solar-only tank, which may require an upgrade to the electric service to handle the instantaneous power load of the tankless.
If your space heaing is with a heat pump there are other options too (desuperheaters, etc.)
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