There isn't any method to choosing a server, just choose one that meets your requirements. If you need more space, go with 2TB drives, or if you have a heavy traffic site go with SSD's. If you need more CPU, go with a better CPU like one of their higher end E3's or E5 processors. Just make sure to put it all into raid-1 setup, and if you do run into emergency issues, call their support. It's much more effective than opening a ticket.