A 9 KW tankless HWH manufacturer claims that it is sufficient for a
whole house at up to 3 GPM.

A 30 gal short tank HWH is about 30" tall and many are mounted over a
clothes dryer on a shelf.

That got me to thinking:

Size - not too important if hung.

Power - both 9 KW.

Instant hot water: - Tank water is preheated. The tankless requires
0.5 GPM to begin heating.

Cost - Conventional HWH is cheaper.

Longevity - Tankless is supposed to last twice as long.

Replacement elements - Conventional elements about $15 at Home Depot,
etc.
Tankless elements are not standard
and cost more.

Recovery - With 9 KW of heating available in both units they may be
equal.
(You must use up all the 30 gallons before additional
HW is needed.)


Upgrading: Conventional elements are available in 5, 5.5, and 6 KW
sizes.
Therefore a 12 KW 30 gallon is doable and it would
heat faster.

Efficiency: Tankless is slightly more efficient.

Location - I am in Florida where the input temperature is 70*F.