You haven't said what sort of riding you're intending to do, but here goes: I haven't got any direct experience of these bikes but GT are generally well thought of.
Then again there are plenty of good reports on the Fury:
http://www.bikeradar.com/forum/viewtopic.php?p=15898514
Halfords say: "The Carrera Fury won the What Mountain Bike 2010 starter bike supertest, being awarded both "Winner" and "Best Bike £600 & under", scoring 5/5 and described as "top quality hardtail that's great fun on technical XC trains and light and pacy enough to go fast or far."
These reviews are from a couple of years ago but may help, though bear in mind that specs may have changed.
It's worth having a look at the user reviews (green bit at the bottom).
http://www.bikeradar.com/gear/category/bikes/mountain/product/fury-9876
http://www.bikeradar.com/gear/category/bikes/mountain/product/aggressor-9883
You may well read on this forum (and others) that Halfords should not be touched with a bargepole. To some extent this is just snobbery, but in some cases is unfortunately justified – it very much depends on the branch you visit, how busy they are, and the individual that builds up your bike. I have seen bikes that came from Halfords and they have had silly faults, potentially dangerous in some cases. You may not be affected if you're lucky, but do bear it in mind and check that everything is tightened up properly, the right way round (where applicable) and correctly adjusted. Just as an example, a friend of mine recently bought a cheap 'n' cheerful "mountainbike-shaped object" for under £200 and I saw it after it had covered about 10 miles. The rear wheel cone bearings were very loose, the brake blocks were misaligned and the handlebar stem wasn't tight enough. So, if you buy a bike from them, you will very likely get excellent value for money, but if there's anything you're unsure of, get a 'proper' bike shop to give it the once over. Oh, and make sure they sell you the right size frame!!!
Hope that helps.