There is good honest review of the De Gamma 6 at http://www.getoutwiththekids.co.uk/camping/tents/coleman-da-gama-6-family-tent/
I had a wander round an Icarus 500.
I have the Vespucci 6 which is a similar model (currently £216 if you are happy to buy from EU), and if the Vango Icarus 500 is typical of other Vango tents fibreglass tunnels I have owned, will not be as good in wind as the steel poles of the Coleman, even when using the TBS.
One minor thing I noticed is the the De Gamma 6 has more flexible bedrooms compared to the Icarus 500, having 3 doors/2 rooms with a fully removable separator.
The Coleman does come with a 2 year manufacturer warranty.
Vango still 1 year AFAIK.
I looked at the Vango Icarus 500 in Go Outdoors and wasn't really that impressed. They had a Vango Nadina 600, and that really is a nice tent, but out of my price range!
I opted for the Coleman in the end as there's a excellent offer out there for the tent, extension, footprint and carpet for £360. Looking at all the reviews I think I've made a good choice. I just wish they had used blackout material in the bedrooms .