Here's a quick overview of the features currently present.
The SP versions aim at different groups of Dragon® users. SP Standard is primarily meant for less experienced and "regular" users. SP PRO is mostly meant for experienced, more "demanding" users.
As you can see there is some overlap in functionality. The modules in Standard are usually a little bit simpler whereas in PRO they are more advanced and have a higher level of customization. However, PRO also has a slightly higher learning curve.
It all depends on your personal preferences and where you want to use the programs for. It's good to know you can always upgrade to PRO. You only pay the price difference (no extra fees).