After searching a bit on google by the transfer speed of the UMD and the Memory card i would say go for the CSO/ISO.
UMD - Transfer rate: 11Mbps
Memory Card - Transfer rate: 160 Mbps
And the UMD has moving parts, the less they move the less chance is that they break
EDIT: Found
this video on youtube, the top psp is running socom from the memory stick and the bottom from the UMD