The Diablo is a great stem. I run one and it seems to have no flex and never loosens. But if he is building a xc bike it will be a bit too short.

As for all the products you mentioned, they all work great. It is going to be a matter of personal choice. I run a mix of Race Face, Shimano, Thompson and Easton on one bike. As mentioned before, check out Thompson. Very good equipment.

As for looks I would say either the Race Face stuff or Thompson.

Just know that if you get the Octalink you have no choices on what bb you can use. Only the Shimano will work. Where as the Race Face you can get a bb from many diff companies that will work.

Slainte