Why Dell PowerEdge vs Cobalt RaQ 550 or Sun Fire V6x servers?

I am an IT Manager and we are looking to replace our Cobalt RaQ 4 servers.  While these servers have done the job, we need a bit of an upgrade.

I have been researching the Cobalt RaQ 550 servers and the Sun Fire V6x series servers.  It appears that the Sun Fire and the PowerEdge 1750 are similiar.

Are there any compelling reasons to chose Dell over Sun, other than brand loyalty.

And no, this is not a commercial or any thing like that.  As we need to replace 10 servers and probably add 5 more, I want to make the correct choice up front.