Razor already linked to
Vadim Verenich's analysis, that is probably the best out there. Sorry that it does require DNA Forums registration to view, I have summarized it a little here already, but it's best to view the original.
For interclade analysis,
Nordtvedt is the best, as usual.
Or try your hand with the raw data... the largest collection I know of is at the
I2a Project.
I don't claim to have the answer to my own question... Hopefully, I've been able to communicate what I know, what I don't know, why I made the choice in my own poll that I did, and what it would take to swing me to another choice. I want to hear what others have to say. To be honest, I was expecting more to defend the Sarmatians, and fewer to defend Paleolithic continuity.
Maybe it would be best to rank the choices from most to least likely. I would put them like this:
(1) The Slavs (best based on what we know about history and I2a-Din)
--gap--
(2) The Sarmatians (also fits history, I suppose, but poorer correlation to I2a-Din so far... we'll need more Eastern I2a-Din to lend credence to this theory)
--gap--
(3) Paleolithic continuity (I think that dating it back this far is impossible, so this needs a demonstration of a recent double-bottleneck in the Balkans to explain modern I2a-Din... discovering close, native Balkan relative clades of I2a-Din would help this one)
(4) Sea Peoples (would explain the distribution relatively cleanly, but needs some actual archaeological accounting and a badly wrong date estimate based on STRs, or a double-bottleneck)
--gap--
(5) The Early Indo-Europeans (can be safely discarded IMHO... wrong everything)