esxi 5.1u1, mb with 2xsata3 ports, want add SSD cache but which port?

Good morning to everyone,

this is my first post and i'm not english-speaking, so i'll try to explain my question in the best way.

We are running esxi 5.1u1 on a small server with desktop components:

- i5-3570k                                        [could get i5-3570 non-K]

- z77 motherboard (msi g43)             [could get Q77 board from gigabyte] 

- 16gb ram ddr3 1600mhz non-ecc

- 2xHDD western digital RED 2TB each, non-raid

- backup on NAS

and about 10vm between linux and windows hosts (usually only 6 are up for normal production[AD, FS, PBX), to manage a network of 4 workstations.

We are wondering to add a 120gbSATA3 SSD as caching, using build-in feature of ESXI 5.1u1, but we have a problem on choosing the correct disk configuration since we got only 2 sata3 ports.

1. HDDs are sata3 and actually connected to the 2 sata3 ports (intel controller);
2. we got other 4 sata2 ports on different build in controller;
3. we can't afford a raid card at the moment;
4. should we put the ssd on sata2 to benefit the cache speed, or the only way is sata3?

Thanks in advance.