It depends on what you want to do. I used to MTB quite a bit some years ago and then took a break until Nov last year. I dragged my old bike from the garage and started MTBing again, so in some ways also a fresh start. I realised that I wanted to ride relatively competitively, but also in comfort. My old bike (a decent enough 26" did not give me this, so I upgraded to a 29er dual suspension. My ride is comfortable and I can realistically aim to finish in the top 30 - 50% of the field. I would not recommend that you buy a 26er and further recommend you consider a dual suspension. You really need to decide what is important to you and then decide.