I haven't developed a good opinion on defence position, but for defence selection,
this is an excellent resource. If you scroll down to "Year-Specific Statistics" on that page, it has data on the number of defences damaged of each type throughout the year. Based on this data, the statistically hardest to breach defence picks are:
Cheval De Frise
Ramparts or Moat -- data inconclusive as of week 3
Drawbridge
Rock Wall
For the argument between the Ramparts or the Moat, I tend to believe that the Moat is more likely to cause damage to enemy robots (i.e. comms lost, mechanisms breaking) whereas the Ramparts takes longer. If picking blindly, I personally would pick the Ramparts.
As for the Sally Port vs the Drawbridge, the drawbridge is statistically crossed much less often. In spite of this, teams have been picking the Sally Port more than double the amount that they pick the Drawbridge. In the end, it comes down to preference and scouting.
Of course, those defence picks are based solely on statistics. There will definitely be cases in practice where certain alliances will be completely trumped by the Portcullis, or for which the Drawbridge is easily managed. But if it comes down to a blind pick, those are my recommendations.