I have the Oasis, and mainly fish with my 9-yr old son. It's great. It moves better than the Outfitter, but.. it is a bit more tippy from side-to-side. My son is big enough to help with the pedalling, and knows to keep his head midline. If your planning on having younger kids that you might have to scoot forward to tend to them.. maybe the more stable Outfitter might be better. It's not as fast gliding thru the water, but it's wider, more stable, and roomier. It's probably a better fishing platform also. (as far as tandem kayaks can be, for fishing)
I know nothing about that Big Tuna.. but I will say that with kids, you have to always expect them to poop out on you, and not pedal or paddle

, .. so you may have to plan on doing all the work from time-to-time... Having both pedals & paddles make it nicer. Just a thought