One issue with disk spanning is you have no control where the data goes. For example, lets say you have a 5MB powerpoint file. Half of that file may be in one drive, and the rest in the other. This will speedup saving and loading the file. On the other hand, if one of the SSD fails, your data is toast. This is true not only for one file but for all the data in the LVM partition as a whole. Say you create a single 750 GB partition spanning the 250 GB SSD and the 500 GB SSD using LVM. One of the SSDs fails. You will have half a partition. There is no way to recover any data from half a partition in the working SSD.