I'm using Zenfolio and haven't noticed any issues with mac compatibility. I use Aperature as opposed to iPhoto, but I honestly don't see how that would make any difference whatsoever.
Anyways, my experience with Zenfolio has been great thus far. I find the interface is generally very clean and userfriendly compared to a few others I've tried (Flickr, Smug Mug)... of course this is a matter of personal preference. The one thing Zenfolio does not have (yet) that some other sites have is the ability for visitors to leave comments... this is something that zenfolio is sorely missing... although from what i hear, it is in the works.
I say if you're worried about compatibility.. then don't worry about that. You'll have no issues. If you're having trouble deciding between the different sites, I suggest using their free trials to help in your decision.
There is a link in my signature to my Zenfolio page. Have a look and let me know if you have any questions. Now for the shameless plug...
If you ultimately decide to go with Zenfolio, you can save $5 when you sign up using my referral code: 1JP-1S8-E6Z