Hi, Mike,
I went the HD route, as I run multiple VM's at one time, a VM Lab and Nested VM's often. I use a Lenovo Thinkpad W530 with 32 GB of Ram, i7-3740 Quad-core CPU and 2x 1 TB HGST 7200 RPM HD's. For fast Boot, I use a 32 GB mSata Cache (24 second without login and 27 seconds with login enabled) There are a couple of vendors who make 1 TB SSD's but for me its not worth the price right now, $500 for the SSD vs $75 for the size HD. I run Win 7/8.1, Several Distros of Linux, Win Server 2008 R2, Win Server 2012, ESXi to name a few. At any given time, I may have 19+ VM's on the HD. With the amount of Ram I'm running, the VM's are always very responsive.
A lot depends on the size of your VMDK files, it they remain somewhat stagnant and don't grow too much then the SSD may be the best choice, however, if they tend to get very large you best option may be a 1 TB HD.
Steve