Given the opportunity I would go for both!
The extra CHIP will allow for games that need 1MB or more CHIP and the Fast RAM will give you a 33% speed boost as well as the option to RUN WHDLoad.
so if you can get it - get both =)
all you need now is a form of Harddisk and you sir are ROCKING!
you could infact just get the 1MB extra chip as finding an A504 is going to be a monumentus task in itself - so instead I suggest ninja'ing over to AmiBay and look up tomthul's most awesome 2MB FAST and IDE adapter - a great bit of kit!
you will have a floppy boot disk that then hands over to the IDE device on the internal 33pin chain - great for Compact flash and a SFF CDROM if your hackery and imagination can take you
if you want a sneaky peeky at what it can do for your A500+
heres a
[ Show youtube player ] I made at Bletchley park 25th Amiga Aniversary (setting up) =)