A ram disk requires you to set up the ram disk each time you reboot. This is the fastest performance option but the most cumbersome to setup and maintain.
SSDs in raid do not give a great performance boost over SSDs. The reason RAID gives HDDs great performance enhancement is the interleaving of the read/writes. You approach double the speed because you have two mechanical spinning devices reading. With solid state there is no great improvement for controller read/writes.
A HDD with SSD caching will speed up the Hdd but will not match the SSD in speed.
The best solution with cost in mind is straight SSDs. The 256GB models are faster than the smaller ones.