One big or group of little servers+SSD or hdd sas

Imagine we have a enterprise hp server with virtualization ability and we don’t need to pay anything for it now.
1.A big multi core server is better or a group of little virtual servers?
2.ssd or hdd?! how much is important in lightning? An array of sas 10k or 15k in raid5 is good enough? or ssd is very important and performance of lightning will decrease so much?