List of lists? Data frames? (Or other data structures?)

Hi, I'm faced with the following problem and would appreciate some
advice.

I could have a data frame x that looks like this:
         aa          bb
a        1           "A"
b        2           "B"

The advantage of this is that I could access all the individual
components easily.  Also I could access all the rows and columns
easily.

Alternatively, I could have a list of lists that looks like this:

xprime <- list()
xprime$a <- list()
xprime$b <- list()

xprime$a$aa <- 1
xprime$a$bb <- "A"

xprime$b$aa <- 2
xprime$b$bb <- "B"

etc.

If speed is important, would a list of lists be faster than a data
frame? (I know, for example, that scan is supposed to be faster than
read.table, but I don't know if that is related to issues with data
frames.)

My problem with a list of lists, though, is that if I want to access
all the bb subcomponents, a naive method like this one failed:

y <- c( "a", "b" )
xprime[[ y ]]$bb (Does not work)

So to get all the bb subcomponents I seem to need to loop, which may
slow things down (presumably).  But maybe people here know of a way.

Finally what would be the "best" way given the constraint of quick
access to all rows, columns and individual components?

I'd appreciate your thoughts and comments.  Thanks very much.