I'd go with the 300g. The difference of it being 6" wider compared to 12" longer isn't a comparison to me. A wider tank would be nice, but like I said...it's only 6" and it doesn't seem like that big of a difference when it comes to turning room. Whereas, the 12" legth gives it more swimming room.
Your friend will have to also consider future fish that they may keep. If aros are going to be a staple then go wider, but if you move to smaller monsters then go for the length.
we have blue arawanas so I would go for the longer tank you know so that they can really build up speed before trying to shoot out the top like blue rockets you have silvers you know what im talking about...loolololol
I'll just play along, it seems like you are always getting something exciting and new???? Hmmm.......... I usually pick wider so I guess I would like the 280, but the longer one will actually look MUCH bigger to me!! I guess it depends on where I am gonna keep it in my place and whats gonna live in it???