May depend on what functionality you need. Have a look at the comparison chart to begin with click here. Some of the extra functionality with the more advanced versions may be had with free alternatives.
Another aspect to consider is should you go for an OEM or retail version? Go for OEM if you won't ever be uninstalling it to install on another system in the future - go for the retail version if you will.
If you are, or have a student in the home, you could be eligible for a 'student license'. This could work out the most economical of all click here (read the FAQs before purchasing click here).
How many systems are you going to install it on? If 3, then a 'family pack' for ~£130 delivered click here could be the one to opt for (this may not apply to you, but I'm just mentioning all the available possibilities). G