move 15 in Kasparov vs World

So I ran a few programs on this position.

Spike and Toga II like ...Ra8 and Rybka likes ...b5. "The World" played ...Ra8 but Kasparov feared ...b5.

Spike 1.1

info depth 24 score cp -32 pv h8a8 c1e3 e7e6 e3b6 f6d5 c3d5 c4d5 d1d5 e6d5 f1b1
a8a4 b6e3 d7c7 b1d1 g7b2 a1b1 b2d4 e3d4 c6d4 b1c1 c7d7 d1d2 d4e6 d2d5 a4a2 c1d1
a2a6 d5d3 hashfull 1000 nps 527191 time 297483992 nodes 156831064410 tbhits 0

Spike 1.2

info depth 24 score cp -15 pv h8a8 c1e3 e7e6 e3b6 f6d5 c3d5 c4d5 d1d5 e6d5 f1b1
a8a4 b6e3 d7c8 g2g3 c8c7 g1f1 d5d4 b2b3 d4e3 b3a4 e3f2 a2a3 g7a1 b1a1 f7f5 f1f2
d6d5 hashfull 1000 nps 564996 time 209331429 nodes 118271594633 tbhits 0

Toga II 1.2.1a

info multipv 1 depth 25 seldepth 82 score cp -12 time 342210312 nodes 180871282838 pv h8a8 c1e3 e7e6 e3b6 f6d5 c3d5 c4d5 d1d5 e6d5 f1d1 g7b2 a1b1 b2c3 d1d5 a8a2 b1d1 c3e5 b6e3 a2a3 d5b5 d7c8 d1b1 c6d8 g2g3 h7h5 e3b6 d8c6 f2f4 e5f6 b6f2 c6a5 g1g2 a3a2

Rybka 1.0 beta

info depth 23 score cp 0 time 351281642 nodes 1461490788 nps 4260 pv b6b5 f1e1 h8c8