Roadies first mtb!

Hi

I've done a lot of road cycling over the past few years but fancy trying mountain biking for a bit of a change, my problem is that I'm really struggling to decide what to buy!

I'm 6'2", 80kg and will probably use it almost primarily for some basic trails and as my skills progress move onto harder trails so something with a good frame that could be upgraded would be great.

My budget is around £700 although I could stretch to £800 if it would make a big difference, and it has to be bought from Evans Cycles as it is through a cycle to work scheme.

After a bit of research I found the Trek 4900D and the Specialized Rockhopper Comp! Are these good bikes for the money? If so, which is the better buy? Are there any other bikes I should consider?

Many thanks to anyone who helps!