Depends on the purpose (Score 1)

It depends on what you are using the computer for. I use one of my desktops for:
(a) DHCP/ICS/etc... (doesn't need hdd)
(b) to run Windows programs on (is slow because of Remote Desktop and the slow processor, so slow hdd isn't noticed)
(c) print server (doesn't need hdd)
(d) permanent storage/ backup (is not frequently used, so being a little slow is ok)
(e) CGIproxy (low hdd usage)
(f) Streaming Audio/Video (has set, continous hdd usage that any hdd can handle)

The advantages of having a 2.5" hdd can be quite distinct, for having virtually no speed difference:
(a) very low noise (virtually inaudible)
(b) low power consumption ( ~2.5 watts ) so it costs less to run
(c) size difference