I've paddled both the Epic V8 double (in the back) and the Nelo 600 double (in both back and front).
The main advantages of the Nelo 600 is that it is shorter than the V8 double and it can be steered from either the front or back (or both!). So if you want to put a novice paddler (such as your child) in the double, putting him or her in the front to watch what they are doing and steering from the back is a major advantage.
The disadvantage of the Nelo 600 is that it is a wider catch in the rear the rear than the V8 double. So you will have to adjust your stroke moving from a single surfski or from the front of the surfski to the back.
I suspect that the V8 double is a bit faster than the Nelo 600 because of it is longer and narrower.
Both surfskis are great fun. You can see me paddling the V8 double here (
) in the back and the Nelo 600 in the front here (
).