Arrivng GUA

You can just walk out of the airport and choose someone who has "Antigua" on a sign; the cost will be $10 per person. You may have to wait a few minutes for the van to fill and a few minutes on the other end if others are dropped at their hotels/destinations in Antigua before you are. Be sure to have your hotel address handy; they'll say they know where it is to get your business but that hasn't always the case for me.

You can hire a taxi on the spot; cost might be $25-35, depending on your negotiating skills (shared, not per person) or you could arrange something in advance; here are 2 transportation providers I’ve been pleased with in Guatemala for share shuttles, private transfers, and flight arrangements:
http://www.adrenalinatours.com
http://www.rainbowtravelcenter.com

PS No, I don't think you can count on making it out of the airport half an hour after your flight is supposed to land or on the shuttle waiting for you, but you won't have trouble finding a ride.

I was thinking about heading to Panajachel on the same day and the shuttles go through Antigua...I think there is one at 2 pm that goes directly to Panajachel...a taxi I've heard is around 80 dollars to Pana

Yes, the share shuttles go through Antigua. If you hire a taxi or arrange a private shuttle in advance, you can head straight to Pana, likely saving an hour or more. $80 would be a good price for a private ride to Pana ime.