d5
2 votes for d5 (Hueristic, boolberry)
I was expected 8.a4 so was happy to see 8.c3 instead
OP I hope you don't mind but I converted the game into PGN below. It should make it easier for new players to analyze the game without having to find a physical chess board.
Game PGN: 
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7 6.Re1 b5 7.Bb3 O-O 8.c3
For anyone new to PGN, just copy the code above and then paste into your software of choice. Here is a free online option:
http://www.chess.com/analysis-board-editorIf anyone wants to easily review the first game, here it is:
Game PGN: 
1.Nf3 d5 2.c4 e6 3.g3 Nf6 4.Bg2 dxc4 5.Qa4+ Nbd7 6.Qxc4 c5 7.O-O a6 8.d3 b5 9.Qc2 Bb7 10.Nc3 Be7 11.Bf4 O-O 12.Rfd1 Qb6 13.a4 Rfd8 14.a5 Qa7 15.e4 Rac8 16.h3 Nb8 17.Be3 Nc6 18.Qb3 h6 19.Ne2 Qb8 20.Rd2 Nb4 21.Bf4 Qa8 22.Ne5 Nd7 23.Nxd7 Rxd7 24.Qd1 Rcd8 25.Nc1 c4 26.dxc4 Rxd2 27.Bxd2 Bxe4 28.Bxe4 Qxe4 29.cxb5 Nc2 30.Nb3 Nxa1 31.Nxa1 Qd3 32.Nb3 Qxb5 33.Qc2 Qe2 34.h4 Bf8