no25, no36 and no112 are worth a look too, 6" same as the no6 but have quick release and are heavier construction, no25 was always grey cast iron, I think the no36 and no112 were steel originally but changed later to ductile iron. Good condition and the right price is probably more important unless it will be worked very hard.
I have a 35P (5", steel vice, plain screw, no QR). Light (for a vice) strong & has a very good jaw opening. Does pretty much all I need.
The heavy 6" cast beasts are I think too big unless you're slogging away on very big stuff (which I don't)
Thankyou all for informative answers something to think about I currently have a 110 steel QR which is a really good vice just could do with something that opens out further.