Noreen:  I can't answer about studies, although I doubt there are any.  I
can comment from actually seeing the differences between the bottle nipple
and the nipple shield in action.  Although both are made from silicone,
there are significant qualitative differences.  With the nipple shield,
the "teat" part, i.e. what fits over the nipple, is firm, but the part
that fits over the areola is very soft and moldable.  So when a baby
nurses with a nipple shield in place, presuming that positioning and latch
are correct, the back of his/her tongue is cupped around the firm teat,
but more importantly the front of the tongue is on the soft part over the
areola, which in my mind provides the baby with a much closer experience
of nursing at the breast.  By contrast, the bottle nipple (Avent's only
claim to fame is a broader base, requiring a wider opening of baby's
mouth), is firm all over, so it really doesn't give the baby the sensation
of a soft areola to latch on to.

My personal belief is that if a baby is not latching well, breastfeeding
with a nipple shield is far preferable to bottle feeding with any nipple.
If a bottle must be used, I may recommend the Avent because of its wider
base.  But no matter what any of these companies say, no artificial nipple
in any way comes "close to mother".

Hope this helps.  Alicia Dermer, MD, IBCLC.