the two drugs work in different ways.  tim hopefully will correct me here
but from what i understand, letrozole affects the body as a whole more
because it blocks the entire body's ability to make estrogen, rather than
just the breast.   i could be wrong tho.
regardless, i take letrozole.  i have been on it for one year.  there are
few side effects, but occasional pain and nausea are among them.  i looked
up the info on novartis website for femara (brand name)  when i felt i was
having side effects that were not described by my onc.  they are all listed
there.
but i can say, it's not very oftern that i feel nausea.  every once in a
while for a short time.  never longer than an hour or so, and usually just
for a few minutes.  everybody's different though.  i would go read about it
and  ask the doc next visit about any concerns.
good luck with it.

-c.