Another downside to the tankless approach is the true fill time.
In winter the incoming water temp in NC will be about 40-45F (depending on location), so you're looking at something like a ~65F temperature rise. With a 199KBTU/hr condensing gas heater you might get 195KBTU/hr out, which translates to (195,000/65=) 3,000lbs/hr, or 50lbs/minute, which is (50/8.34=) 6gpm. So for the 40-50 odd gallons you'll need it'would take about 7-8 minutes.
With a tank heater your fill rates are limited by the impedance of the plumbing, and in most cases you'd get it done in something like 5 minutes provided you're using 3/4" plumbing both to and from the water heater.
A 50 gallon condensing tank heater can get you there provided you have a big enough burner under it. The 50 gallon condensing Vertex comes in both a 76KBTU/hr (73K out) and a 100KBTU/hr (98K out)version. The 76K burner unit puts out 1200 BTU/minute, so with a 65F rise that's 18.5lbs or 2.2 gallons per minute, so during a 5 minute tub fill as the hottest water is leaving the top of the tank, it's capable making another 11 gallons of 110F water, but it won't be evenly distributed, it'll be mixing in with the water at the bottom, diluting the 140F water down by quite a bit. If you only needed 40 gallons of 110F water it would probably make it, but if you need the full 50, probably not. The 100KBTU/hr version would making about 15 gallons in 5 minutes, so if you started out with 140F water in the tank you should be able to get the tub filled in 5 minutes or less without the water at the tap dropping below 110F.
Another downside to a tankless is the size of the gas plumbing necessary to get that many BTUs reliably to the burner, which in most houses would require 1-1/4" pipe (maybe 1" if it's less than 30' from the regulator with but a few ells). Depending on what other gas burners you have in the house, you may need to up-size the regulator too. The bigger-burner Vertex is still only half the size of the tankless, and if you're within 40' of the regulator you'd be able to use 3/4" gas plumbing. If it's looking too close, with a handful of ells you'd only need to upsize to 1". The 76K Vertex would be fine with 3/4" no matter what.