If you are worried about eviction, neither a medium citadel nor a POS you anchor in an empty wormhole are likely to attract much attention unless you neglect to fuel it. Citadels are FAR better in terms of QoL.
In terms of defense, the tedium of taking down your structure for little reward is your first defense against bigger groups. However the reinforcement mechanics are much better for citadels since you get 2 timers and can at least control the time of day the timer will come out.
The next big defensive advantage from these structures is strong ewar options. Both citadels and POSes can be fit with effective ewar, but an Astrahus is generally more defensible unless you are planning to get a large faction POS.
Finally, a bit of advice for moving to wh space is to day trip a bit first to get comfortable. Picking the right system for your group based on class and statics is important for providing suitable content. I usually recommend C2-C3/HS for new wh groups getting started.