Don't get me wrong the Islabike has been great but it came up short when we were living in Scotland and riding some of the trails around Glen Nevis. The geometry is just wrong for anything beyond forest type roads. My older son was able to jump his Trek and ride it down some fairly bumpy terrain but the Isla didn't have the handling for that. To be fair it's probably not designed for that but if by off road you mean forest tracks and well surfaced trails, it's absolutely fine, I wouldn't hesitate to recommend it and you can wave goodbye to him up hills.
The Trek is heavier but not so heavy as to make it a chore to ride, in fact it's not bad and has been a great all round bike. It's certainly had plenty of abuse and stood up to most of it. Glen Nevis finished off the front suspension though. It's also highly adjustable in terms of reach and pedals. There are two pedal positions on the cranks and the stem rotates backwards to shorten the reach, though the new ones look different, so you'd have to check that out. Because of that he's had it from 7 to 12, though I'm really pushing it making him ride it now as he can ride my bike.
I like both of them but f I was forced to choose, I'd say the Trek is the most flexible and more all round capable. The Kona by the way is similar to the Trek but less good on the road but it's a lot cooler than both in kids eyes.