A top box is secure, convenient and makes like Givi will be very well made and waterproof. They are not cheap but the advantage is that is can be transferered to your next bike with just the addition of a fitting kit for your mew machine. The disadvantage is that is puts weight up high and beyond the back wheel.
Saddle bags, as in the soft variety put the weight lower down but are not secure. You'll need to make some provision to ensure that they do not scratch your paintwork and you will need to ensure that are securely attached to your bike so that they do not move when you are riding.
For light loads over short journeys many find a rucksack ideal for the job. Of course, that can't be left on the bike when you park up.