Everyone has their own opinion on tents because we all like different things.
I personally will be buying the Vango Diablo 600. The 2006 model is being replaced with the 2007 model so you should start to see the prices fall. Now a downside with this tent is in the sleeping compartments. It's been said that they begin to sag whereas the 2007 model has rectified this problem with an extra supporting pole.
The 600 has 2 sleeping compartments which are 3 berth each. You can get the 900 with 3 x 3 man sleeping compartments.
I also looked at the Gelert Tahoma 6 and the Khyam Breckenridge. I haven't seen the Tahoma erected but I have been to see a Breckenridge and I just didn't like it as much. Smaller bedrooms, porch and living room.
Going to the show at the NEC will help you make your choice but there'll only be about 30 tents there which might not help make your mind up so if you can wait, there's another bigger show in March.
How did you win tickets btw?