I am currently having my master bathroom remodeled, and I have a 6ft whirlpool tub and a new spa shower with multiple shower heads and body jets.

Now what I am wondering is...I currently have a 40 gallon water heater that is only a few years old, and my plumber is recommending that I install a new 50 gallon tank to just the new tub, and leave the 40gal for the rest of the house....would that make sence? or would it be better to plumb them in series? It just seems like the extra capacity of running them in series would make more sence...but I am not a plumber...so please help me figure this out....thank you

I would not connect them in series, but I would interconnect them with valved piping, so that when one fails, you can use the other for the whole house while you wait for the replacement to be installed.

Hey, LivDoug,
Just a thought... when my nephews moved in (to go to local college) our hot-water supply was NEVER adequate. We added a "demand" water heater. It attaches near the tub/sink/whatever. They are available in gas and electric. Some only heat cold water but some models boost already warm water by many degrees. The makers tout the energy saving since water is heated only during use, but the really big plus is an unending supply of hot water.

I have looked at those, but the problemwith a tankless water heater is that even when looking at the more expensive models, according to their specs, would have a hard time flowing more than 3 gallons per minute during our cold wonters here in northern NY. And I am worried that that will not quite cut it :O/

I'm sorry but I really don't have any useful advice for way up north (in Yankee land. I live in Mississippi and the ground temp is around 60 degrees year round so only needed a relatively little boost.
Good luck!
Aunt Connie