More praise for the islabikes here (we're working our way up through the range - eldest currently has the beinn 24), and criticism of the Trek.
Personally, I wouldn't bother with the schwalbe marathons unless you feel you are likely to be in a puncture prone area - our experience with the tyres supplied as standard has been fine - I don't recall any punctures to date).
Also, if you want to change to knobbly tyres, I've found the mudguards also have sufficient clearance for the Continental Explorer tyres (which wasn't the case for the beinn 20).
To me the Islabike wins over the Trek in every respect. Both bikes are similarly priced, yet you get much better quality from the islabike.
The trek and the ridgeback are similarly specified bikes so the comments below will probably apply similarly to both).
If you don't mind my opinions on the Trek - it will be much heavier than the islabike as it has heavy, unnecessary front suspension and heavier steel components eg seat post, handlebar stem.
To me, weight is a key issue as each pound /half kilo extra here makes a huge difference, where it may be fairly insignificant for a 75 kg adult, the extra weight has a greater impact on a smaller, weaker child (eg my 25 kg 7 1/2yr old) - if you see what I mean.
The triple chainset at the front isn't necesary and again this and the derailleur and shifter will add weight.
The trek frame doesn't have space for a bottle cage, the islabike does, and has braze-on bottle cage mounts.
The islabike has quick release wheels which is nice, and makes it quicker to remove the wheels if you need to throw the bike into/onto a car. The quick release is kinder to the paintwork than nutted hubs with spanners
The islabike has a sealed, maintenance-free bottom bracket, the trek has an 'adjustable ball bearing' - the sealed unit won't need adjusting.
We've had excellent customer service from Islabikes, we did have a trek childs bike a few years back and when I contacted Trek about mudguard compatibility I got a useless (and snotty) response.
Finally, as people have noted elsewhere the quality of the islabikes shows in the way they hold their value.
Good luck
Richard