Just taken old plugs out of my mildy tuned WRX MY96 import & they were NGK PFR6G???

I have checked all the old threads & most recommend PFR6B for stock or NGK PFR7B for tuned applications. Just a bit of advice which would be much appreciated to a newbie:

I can get a new set of Denso Iridium SK20PR-L9 off my mate for nothing or would I really be better off buying the PFR7Bs (would like to save £40 if there's no difference other than the gapping of 7 compared to 9)?

Also, just wondered if anyone knew what the difference was in the last code of the NGK plug being a G instead of a B in the old plug (I know what all the other codes stand for but not the last one)?

Thanks.