I would have Macgill.
I am an advocate that someone is rarely going to do much better in Test cricket than FC cricket and Hogg's FC bowling record is poor; with a bowling average of 40.33. Although his batting record is good; I do not feel either his bowling or batting is up to all rounder status; especially for a team as good as Australia who have Symonds and Watson already knocking at that door. His wickets are mainly earned when batsmen try and force the ball in the middle overs and either catch the edge or miss the googly completely, or get the leg in an awkward position, trying to push singles. I don't even think that one can say that Hogg has turned a corner in his bowling either; taking into account his List A stats, his ODI performances have been just what was expected; especially since he is bowling to many batsman who would have only faced him once or twice before.
MacGill on the other hand is proven at test level with an average of 27.20 over 40 test matches. He spins the ball a long way, both ways and Muralitharan has showed how such a tactic can work wonders in Sri Lanka. MacGill is one to take big hauls and I also feel that with a potentially inexperienced pace attack; an experienced operator, capable of ripping lineups apart could be infinately useful.