I think Itachi is the one that's going to end up with the Kyuubi mission. There are a few reasons that back this up.

1) Itachi was sent to get Kyuubi with Kisame before.

2) Itachi, when breaking of pursuit of Kyuubi because of Jirayia, indicated to Kisame that they do not have to hurry in getting it. The language indicated that he was talking about himself and Kisame coming back later, rather than Akatsuki as a whole.

3) It would make sense for Itachi to come after Kyuubi. He is a) strong and b) from Konoha. The latter would make him suited to going after Naruto, as he would know the kinds of techniques he could have been potentially taught. He has been through the same village afterall and probably knows the most information about it than any other Akatsuku member.

Of course an interesting point is made that there seem to be more Akatsuki members than hidden villages. This may indicate that is potentially more than one member per hidden village in Akatsuki. If this is the case then point 3) could be challenged if there is another Konoha Akatsuki member in existence. Oro was there as well afterall so it isn't toofar fetched.

As for Deidara being the weakest as he got the Ichibi, I'm not too sure about that. Kisame didn't seem all that to me either.