Depends.
I think it would be nearest or easiest to get to where you do most of your work.
Sometimes being stuck in traffic trying to get to the merchant that's slightly cheaper on price can be false economy.
If you fit boilers you need to be competive on price, its a very competitive market, so ask them for their prices on the boiler you like to fit.
If you mainly fit bathrooms its not so competetive so you could be more relaxed about price, (but not too relaxed).
There also independent merchants who have say 1 or maybe a few more branches and they some times will do a lot more for you and go the extra mile, than the local branch of one the big national chain of plumbers merchants.
But if your working a town away from your usual base then its nice to be able pick up something from the local branch of a national chain.
Ask for prices of tube both copper and plastic, and the prices of you favourite boilers and then open three accounts and after a couple of years you'll know which one suits you best.