It all comes down to what you want to do with it. First of all if you have a stable and pre-tested machine that you want to use for storing large amounts of data, backups etc I would put as many HDDs as you can and colocate it. Datacenter will charge you a fortune for each HDD and for time it takes to upgrade their dedicated servers. However if you don't want to bother with packaging and shipping or you need a machine online asap ordering a dedicated server is the best way.