They do spook easily. While there are probably few over the 2ft mark in the hobby it's not because they don't get much bigger just that no one has been able to keep them alive for an extended duration in typical home aquarium environments. Personally if it was me and the 300 gallon was the "permanent" home with no plans to go bigger I would probably avoid both and stick to something that will stay smaller like a Bolt Catfish, Vulture Catfish, Juruense, or even a Tigrinus. Also tank mates matter when looking at any of these catfish, don't know what you're keeping but could become expensive snacks for any of these Brachyplatystoma species fairly easily.