Bike Radar did a test, about as scientific as these things get. Same rider, same conditions, same track, 3 different wheel sizes. Their conclusion was the difference between 26″ and 29″ was negligible. 26″ took the descents, 29 took overall average speed, 27.5 was the all round loser. So, according to this one study… I can safely suggest 29″. Needless to say, there are other factors in play. Not to mention the placebo affect of a new bike being ultimately ‘better’.
Bike Radar Video
[edit]I forgot to mention… I ride a 26″ bike, so… I can only really supply the link.My opinions are worthless in the real world.[edit]