Where's a big crystal ball when you need one??
If you think interest rates are likely to rise in the next few years, go for a fixed rate - lots of good deals about at the mo, enabling quite decent lock-ins at reasonable rates.
If, however, you expect rates to stay low, then a tracker will be the best option.
Me personally ?? I'd opt for the fixed rate. You should be able to lock in at a decent rate, for a decent length of time, plus it offers security, insofar as you'll know exactly what you're paying, so you can budget accordingly.
Imagine going for a tracker and then rates rise rather sharpish??
Which one sounds the best option to you??