Hello,
if your servers are using local storage and only one of them will have an SSD, the first thing I'd recommend is getting two SSDs and putting them to RAID1 if you plan to use them on IO intensive VMs (databases) - you'll see a stellar performance from these servers once you migrate them there.
If you have one and only one, you can also use it as a vSphere Flash Read Cache vFRC | VMware vSphere Blog - VMware Blogs, which will boost your read performance for the VMs you choose.