morphyvsfischer: This queen endgame is probably the best out there. 15 Bc2 should be preferred, as should 17 c4. 34...Qc8 35 b4 Qxh3+ 36 Kg1 looks harder for Black to win than in the main line, if black plays correctly.
40...a5 will win. The choice made can become a draw after 45 Qxa4 Qc1+ 46 Kxh2 Qxc5 47 Qa2 Qe5+ 48 g3 Kg8 49 Qc4 Qc7 50 Qb5 Qa7 51 h4.
Even after the bad 45 c7?, White may still be able to draw, for instance 54 Kg1, 67 Qd8!, 87 Qa1!
85 Qg1+ Ke2 86 Qg2+ Kd3 87 Qf3+ Kd2 88 Qf4+ Ke2 89 Qc7 Qe4! gives Black mating threats, while his f pawn does not hang.
88 Qg1 f4! 89 gxf4 (Qg2+ Ke3 90 Qg1+ Kd2 91 gxf4 Qe2+ is a winning king/pawn ending) Qc2+ 90 Kh1 Qe4 91 Qg5 Qb1+ 92 Kh2 Qc2+ 93 Kh1 Qc1+ 94 Kh2 Qxf4+ forces the queens off, but is still a better defense.
Cross checks abound here, and equal material that still wins for Black is also quite common once the queens are forced off.