Bacrot is a 1.d4 player, but sometimes plays 1.e4, too.
Against 1.d4 Kamsky has been playing the Slav, and the Nimzo or Queen's Indian since his comeback. In the Slav he likes to play modern systems with an early ..a6 or ..Qb6.
Back in the nineties, he played the Grunfeld and King's Indian, however.
I think in a match situation Kamsky is going to play it safe and rely on his most solid opening against 1.d4, the Nimzo or Queen's Indian. If he is behind in the score, he could bring back his King's Indian.
I don't think Bacrot is going to play 1.e4 unless he has cooked up something against Kamsky's super solid Marshall.
Kamsky most of the time plays 1.e4, but sometimes 1.d4. He generally plays main lines after 1.e4, but never after 1.d4, where he likes to play systems with an early Bf4 or Bg5.
Against 1.e4 Bacrot plays the closed Spanish, usually the Zaitsev variation, which would be very interesting to see in the match.
Kamsky has been avoiding the main line of the closed Spanish since his comeback, though. He nowadays plays like
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7 6.Re1 b5 7.Bb3 d6 8.c3 O-O 9.d4 instead of the main line 9.h3.
He also plays like this against the Marshall move order, that Bacrot sometimes uses (7..O-O 8.d4 d6 9.c3).
Of course one reason for Kamsky to play lines like this and the Slav with an early ..a6 is that there is less theory than in other lines. He probably preferred these lines after his absence in order to concentrate on getting his practical part of his game back to where it was in the nineties - and even then he was much more a practical player known more for his toughness than for constant delivery of opening novelties.
It will be very interesting to see Kamsky in a match again, some of his matches in the nineties are classics.