mmap or iomap

hi all,
i m anand,
i m writin a first linux driver for LSI53c875A ultra scsi Controller , in the pci config register ,my leader says that memory map is better than io map (ie ) while requesting for memory region i use the BAR1 which is for Memory.. and BAR0 is IO.
Cud any plz tell me which one is better & why .

thankin u
anand

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