I've moved this over from the Developers forum to a location that seems more appropriate.
To (try to) answer your question, I think you might have a little more luck if you asked about specific features both here and at Kerio's forums.
Personally, I haven't look very closely at Kerio, but I am aware that they've got a beta out which demonstrates partial support for MS Outlook 2011 for Mac. Zimbra hasn't yet demonstrated anything in that direction, although it looks like they're working on it (see
Bug 38631 - Enable snow leopard messaging features, Outlook for Mac 2011) so it's yet to be seen who'll put the feature into production first and better.
For iPad, Zimbra Network Edition includes Exchange ActiveSync, which will work with the iPad's Mail/Calendar/Contacts.
If you don't have Network Edition, you may still be able to use the iPad's web browser, but I've seen some things to suggest that the mobile web client still needs some tweaks to fully support iPad.
iPad and ZCS 6.0.5 Report iOS devices and Zimbra powered comcast.net email Zimbra on iPad question
Yet another option is Aderium Companion--a 3rd party product. You'll want to check with the developer to see if they support iPad.