I think Steadfast offers pooled resources - so, you can resize when you purchase more resources and/or you can spin up more VMs with your available resources
also, Steadfast has a SAN backend, which can and will impact performance (as network based storage is ALWAYS slower then local, backplane attached storage). the issue of course is that the network storage is what enables the HA in the hypervisors. so, you will get faster performance with someone using local storage, but you will lose out on the failover/redundancy of the hypervisors
one thing you might want to consider is to ask steadfast if they have a local storage option for VMs and move your databases over to the local storage model and leave your other services (which arent as IO intensive) in their SAN based model
I don't think you can customize resources with DigitalOcean. DigitalOcean is also just a standard VPS with hourly billing options, no high availability and such. Not sure what Steadfast offers, but it is questionable whether you can consider hourly billing as cloud hosting.
Digitalocean, like most clouds (linode, amazon, rackspace) have a fixed VPS plan style offer. If you want more of anything, you upgrade everything (normally with downtime) to the next bigger plan.
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