There have been a number of previous threads on this forum discussing the pros & cons of the different wheel sizes, so it might be worthwhile doing a search for these as there's a lot of great comments. Some people think 29'ers are the best thing ever, some think they are the work of the devil, and some think it is a load of marketing bo**ox.
Me personally, if I was upgrading my 26'er I would consider a 27.5 but wouldn't touch a 29'er. That's just down to personal preference and prejudice.
As T.M.H.N.E.T. says, test ride is the best way to make your own judgement.