i believe the 74s have FDB motors and the 36s don't. oddly enough the 74s seem a bit louder (probably due to the open nature of my lian li v1000 case - all those holes in the chassis). the 74s are definitely a bit faster on seek and access times but in real world usage, it's hard to tell the difference.
rdgrimes suggestion to run the 2 80s in RAID-1 is better than my initial suggestion and ultimately what i would do if i had that setup of HDs.
slightly off topic but i'm currently shopping around for at least 1 more 250gb/300/or 400gb HD. currently looking at the Maxtor MaxLine III, Seagate 7200.8 and Hitachi 7k40 or whatever model it is. i've almost exclusively used WD drives in the past but am itching to try out the above 3 brands.