in general I am an advocate of 18-20 foot boats for a family of 4. its what I have and i can remember the kids sleeping under the walk around portion of the deck. it was about 2 feet deep and 2 feet high at 5 ft long section of the cuddy cabin.
that said i would pick the aluminum for your needs if they are the two options. a new boater needs to be able to access the front of his boat. and thats's tricky with the Arima. not that it cant be done, just tricky. as you are learning you will go to fast to docks, or logs to tie off to and you will want to avoid collisions. as you get more experienced you will use the motor to get your self out of those situations. but in the beginning you will have an urge to run to the front.... and some tides and river flows and wind combinations will frustrate you to no end and you will just want the boat, any part of it, back to the dock.
have fun figuring it out. and do not rush the decision.