USB is fine for PS1/retro games; should be more than enough for AthenaEnv. The difference only matters for PS2 backups. And there're more options than those two. HDD/SSD, Ethernet, MX4SIO/SIO2SD, MMCE (SD2PSX et al).
That's true; network too. Can play digital backups (off USB/Eth) only by using POPStarter (for those unaware, POPS being Sony's PS1 emulator ripped off the single game that was officially used on). Although POPS isn't really that good (was used, experimentally, only once afterall), USB throughput isn't an issue.