How are you current needs met? How many people are using this? How large are the files they are using? I'm leaning towards the Cheetah since they have been in use for so long and you can get two for RAID 1 for the cost of a single SSD of that size. That way you can increase uptime of your file server (hopefully). Still use the external backup of course. I have no data to back it up, but I dont see that an SSD will make a huge difference with a file server with decent sized files and smaller loads. After you factor in your network and samba/ntfs or whatever you are using, then you will not see maximum drive performance anyway. There is no doubt that in database applications that requires lots of random reads, an SSD would be a better choice. Its not your case though.
An SSD will be faster, but the cheetah is no slouch. I would guess that the cheetah is a bit more reliable too, since it is a proven technology used in critical applications for a long time. In addition, the cheetah will be cheaper for a given capacity.
We are small 4 persons accounting firm. Mostly we use for QuickBooks files and since we are paperless office all the scanning in PDF form goes directly onto the file server as well. Since the reliability is no. 1 factor in my case so looks like Cheetah is the way to go. Thanks