I have both the PIS and Avent pumps. Love them both. I used them for different things. FOr being at work, going out, or "pumping and dumping" after cocktails, I used the PIS. If you plan to pump regularly, the PIS or similar electric pump seems like a much better choice. It's pretty easy, you pump on both sides, and takes about 10 minutes. If you use Avent bottles, there is an adapter so you can pump straight from the PIS to an Avent bottle.
However, for occational pumping, the ISIS would work fine. You can only pump on one side, and to "empty" both sides would take longer than the electric pump. For me, I used it on "one side" while baby nursed on the other. This way, I got a lot of extra milk for bottles. It is MUCH smaller and could fit into a large purse; the PIS is a purse unto itself!
I would get the ISIS first for when baby is first born; it's perfect to relieve engorgement, etc. If you find pumping is for you, go for the PIS.
Kristan