I'm in Scotland, been keeping bees in Smiths last 3 yrs. Planning to get more hives and most here have Nationals. Once I started researching them I read that poly hives have many advantages. (I've lost bees every year with Smiths) then on researching Poly hives I find many positive comments on getting Langstroths. Nothing I'm totally confused!!
If you've had or used Langstroths why are they better?
Put it another way, if you were starting out what would you buy?
I've seen a few poly designs now and see whatever I buy I'll be stuck with buying products only from them
Regarding poly Nationals, that's not quite true.
I think the ModBeek/BeeBox poly 'national' is indeed unique. So lets leave that aside.
The Swienty/CWJ/Solway design only takes 10 hoffman brood frames, otherwise, being standard external dimensions, it is interoperable with standard wooden nationals and anything that interoperates with them. (Pity about the lack of proper bottom beespace over the frame lugs.)
The Paynes and BHS designs are standard 11 hoffman size inside - so bigger outside (500mm square rather than 460mm).
They interoperate with each other and standard wooden parts.
The only two exceptions being that a standard wooden roof won't fit over, and the (unmodified) Paynes floor only takes Paynes broods (or 460 mm - eg wood - parts).
Moving frames from Smith to National can be as simple as putting a (dome-head) screw into each end of the topbar and adjusting for total length and equal extension at each side!
However, if you are happy with short lugged hives (as with your Smiths), maybe you should just go Langstroth.
But Langstroth is not well-standardised, and I am not the one to guide you through the subtle differences in box and frame sizes!