R help - Mar 2007 - Clarification

Hello,

Could someone help me understand why one of these "structures" would
be
preferred over the other?

x <- setClass( "patient", representation( data =
"numeric", fname "character",
                                    lname="character", disease =
"character"
))

jd <- new("patient", data = rnorm(10), fname = "John",
lname = "Doe",
disease = "CB4")


OR

x <- rnorm(10)
attr(x,"fname") <-  "Bill"
attr(x,"lname") <- "Carson"
attr(x,"disease") <- "CB4"

Is this a S3 vs S4 topic?  Anyone have suggested reading for this type of
stuff? I read "S4 classes in 15 pages"; are there other resources?

Thanks in advance !

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