Assisting neighbor - large house (about 6000 SF) ... has two, 10 year old AO Smith 50 gallon natural gas "Energy Saver" (model FPSH 50 236) tanks run in series ...both power vented. One water casing just failed.

I'm not convinced the dual system is the best solution ... options:
1. Replace the one unit
2. Replace both units and maintain the stepped heating approach
3. Replace both 50 gallon with one 75 gallon

What makes sense? If option 3 best bet, which AO Smith (easier sourcing) unit is best application - homeowner will be in house for about 3-5 years.