Hi Matt,
An iSCSI SAN with 6 x 600GB is going to net you around 524 IOPs or roughly 25 based on your current 21 desktops - reasonable based on current desktop load.
A R510 with 12 x 300GB is going to net you 1048 IOPs or roughly 50 IOPs based on 21 or 24 IOPs for 45 desktops as you have suggested.
As VDI IOPs are random and never consistent, you would probably have better success with a single R510 maxed with all 15K drives - but you give up the "shared" storage scenario which in turn limits your DRS / HA and FT capability.
You need to decide what you can and cannot live without - if clustering and other HA / DRS and FT features are not important to your infrastructure, then the R510 would net you higher performance. If you have plans on enabling cluster features such as those listed above, then I would suggest you move to the MD3600 to take advantage of them and add disk as your budget allows.
Again, it comes down to what your "whole plan" looks like, from there, you can start eliminating options.
Let me know if I can be any further assistance.
Cheers!