Rublevsky - Ponomariov
Rublevsky always plays 1.e4. Ponomariov usually answers 1.e4 with the Najdorf, or some form of the closed Ruy Lopez, but he has also tried many other openings.
Rublevsky has quite an original opening repertoire with White, though. Against the Najdorf he likes to play 1.e4 c5 2.Nf3 d6 3.Bb5+, or 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.f3. Ponomariov wouldn't be likely to get a closed Ruy Lopez either, as Rublevsky prefers the Scottish. While these lines aren't the absolute main lines, they still can be dangerous, and Rublevsky is playing them with great success.
Ponomariov - Rublevsky
Ponomariov started as an 1.e4 player, but has been using 1.d4 also quite regularly for some time now.
Against 1.d4 Rublevsky plays the Queen's gambit accepted, or the Slav.
Against 1.e4 Rublevsky plays the Taimanov or Kan Sicilian (1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 Nc6 or 4..a6).
In the Taimanov and Kan, a lot depends on move order and transpositions. After 1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 Nc6 Ponomariov always plays 5.Nc3. Now Black can play many different moves, and Rublevsky has tried 5..a6, 5..Qc7, and 5..d6.
I think Ponomariov feels very home against these systems, and has had great success against them. So I expect we will see Ponomariov playing 1.e4, and 1.d4 maybe only if he is in the lead and wants to take less risk with White.
I think this match will see interesting games. Ponomariov should be considered the favourite. Rublevsky has to try to surprise him.