The Acuvue Oasys is good, but Purevision might still be more comfortable for
somebody else. I'm guessing you don't mean Acuvue2, but the newest AV
silicone lens, spelled Oasys.

Between Oasys, Focus N&D and Purevision there are subtle differences in the
optics and thickness and stiffness that might make one perform better than
another for a given individual. Also deposit resistance, handling, and even
vision can be better with one than another. There aren't any lenses yet that
work for everybody.

These materials are so far beyond the previous generation that the
permeability isn't much of an issue. But the dK/t (permeability) of
Purevision is 110, AV Oasys 147, and Focus N&D 175.  For Acuvue 2 it was 35,
and for B&L's original "30-day" lens it was about 15.

-MT, OD

    Kickbacks perhaps?

I'm going to agree with the other Mike and guess you mean AcuVue Oasys
(which is what I use).

It's not so much that they are thinner (I don't think they actually
are), or that they are softer (a much lower modulus than PV), but that
the edges are rounded off.  The PV edges are sharp, and I could feel
them constantly, no matter how long the lenses were in.  Same with
Focus N&D.

You already got the answer on oxygen throughput, so here are what I
would consider plausible reasons to choose PV over Oasys:

1)  You can leave PV in for 30 days.  Oasys are supposed to be taken
out every 14 days.  In practice, I'm going to say even less for the
Oasys, because the material collects debris and deposits more than
that of PV.  I find that I take them out almost daily during the cold
months (when the air is much drier, and my eyes become a bit parched
when tired), and typically leave them in no longer than four days
overall.

2)  PV are aspheric, which purportedly help people with low
astigmatism (too low to get toric lenses for).  I have 0.25 diopters
of astigmatism in each eye, but did not notice any such effect, and I
still get 20/15 vision with Oasys despite the uncorrected astigmatism.

3)  PV have an optical zone of 8.9mm, while Oasys probably have 8.0mm
(probably, because that information isn't published for Oasys, but
other AcuVue lenses have 8.0mm).  That means better night correction
if your dark-adapted pupils are large enough to make a difference.
Focus N&D are horrible in this respect (stars remain fuzzy blobs), and
I couldn't say with certainty that I noticed any real difference
between PV and Oasys.  Both provide very good night vision with my
eyes.

4)  If you replace the lenses as indicated (30 days for PV, 14 days
for Oasys), PV are cheaper per day than Oasys.  I posted actual
numbers in a previous post, but all I recall now is that the
difference is something near $0.15 per day.  Not a lot, really, but if
you're concerned about expenses, it adds up to paper money over time.
The more frequent removal of Oasys also translates into more lens
solution, if you really want to start counting up the pennies.

5)  Oasys only comes in a base curve of 8.4, while PV comes with 8.6.
If you have flatter eyes, like me, 8.4 is right on the edge of what
can be safely worn.  I probably have to take mine out more often
because of that (not much flow between the lens and my cornea), which
is why I'll be trying the new Biofinity lenses next year, which also
have a larger base curve, and on paper look very much like Oasys
otherwise.