If you're currently running FMS12 on an SSD and switch back to a physical spin disc, you'll notice a performance loss, no question on that. The read/write actions to and from the drives will just take longer. Make sure you've got a good backup plan in place, but in general SSDs have become more reliable over the past few years.
FMS13 has generally been reliable on mavericks server IMHO, also there was a release for FMS12 that made it more reliable on mavericks. If you intend to use webdirect at all though, I might recommend against a mac mini as your hardware platform due to it's limitations.
I would go with the SSDs, and also at least max out the ram. I forget whether or not you can do RAM upgrades yourself in minis now, but it's a cheap investment.
I actually have a solution hosted from FMP Advanced on iMac 3.4 i7 with SSD and 8GB RAM over wifi. It runs fast enough that no one but me is complaining about the speed.
You can do the ram upgrades yourself without an issue now.
I am looking forward to using FMS13 and hope this will help with speed in general. We have no need for web direct right now. If we ever do we will get another machine.
Since backups are handled by FMS it seems like RAID 0 is the way to go to keep the drive access speed up.
RAID 0 offers no redundancy at all so think twice about that. For disk intensive operations like databases, RAID 1+0 is generally recommended
If you insist on using a Mini, I would go with an external thunderbolt RAID box. You can setup RAID with mirroring and the costs should be a bit lower than buying an SSD from Apple.
Generally speaking Minis are not ideal servers except for very small workgroups.