The terrorists will go for the easiest target that gives the highest "reward". Secure one venue (theme park/ visitor attraction/ stadium/ airport/ shopping centre/ etc.) and they will choose another.
On the world stage I think that Madam Tussaud's and the London Eye are Merlin's two most prestigious/vulnerable attractions.
Security at AT is more important for preventing anti-social behaviour, vandalism and theft than for preventing a terrorist incident. If a gunman (gunperson?) got into the park there would be little that a few unarmed security guards could do other than evacuate areas and wait for armed police to arrive.
From the Disneyworld perspective the Orlando shootings has taken news coverage away from their parks natural hazards