What is the difference between them?
Might reconsider re: the national poly. Can you recommend retailer online for a Poly?
Mine are all Paynes boxes .. I like them and they are a nice firm to deal with .. but .. there are several different makes and there are as many opinions on here about the various pros and cons as there are opinions on swarm prevention !
There is a thread somewhere ... which poly ? ... I think .. my search hasn't found it but I'll have another look.
There are two major differences ... Poly boxes that have the same external dimensions as a National (but obviously less space inside) and Poly boxes that have the same internal space as Nationals but are clearly bigger than a National on the outside.
If you have poly and stick to them then it really doesn't matter that much but if you start mixing timber and poly then it can become a tad interesting ... they are all compatible in one way or another you just need to be a bit creative with roofs and crownboards. Mixing different brands of Poly doesn't seem to work very well ... so if you go down the poly route go and have a look at all the ones you think are likely and then decide which you are going to stick with. Although they are all based on National Frames (mostly - there are some Langstroth polys around) they are somewhat different .. some come as flat pack and some are single castings ...
I run 14 x 12 and I like the bigger brood boxes as the bees can overwinter in a single box with all the stores they need in the brood box and there is plenty of room for even quite a big colony without having to have brood and a half or double brood ... but, they are a bit heavier than a standard national when they are full ... Again, there will always be debate about the size of the box you use ... just a matter of personal preference.
The best time to buy hives is in the sale ... unfortunately we are now in mid season and sales are a long way off ...
Sorry ... this is not very helpful .. beekeepers spend hours mulling over the pros and cons of equipment so best get used to it ... I don't think there are any really bad suppliers or designs of poly hives so I don't think you would be disappointed with any of them.