29" = 622etrto = 700c

Get the best of both worlds - if you get a 29er mtb and a 700c cyclocross bike with disc brakes then, dependent on tyre width, your wheels will be interchangeable.

Apart from that my experience of slicks on a 26" bike compared to a 700c road bike - the 26" mtb was quicker to accelerate but slightly harder work to keep at a high speed.