Theres good and bad reasons for both ways of fitting them.
I fitted my Hannibal 1.2M at the rear on a Defender 90 and cut out the back of my Safety Devices rack, you get shade, protection from rain and wind whilst cooking and eating at the back door.
Being at the back still between the side rails its protected from all those lovely overhanging branches and sharp thorns that you get in the Sahel - a consideration as I didnt want the PU cover shredded...Mali is a good example of somewhere that will wreck your cover quite quickly.
With The Hannibal (and I think the Ezi Awn too) I can remove the bottom half of the ladder and move the vehicle if neccesary It does not rely on the ladder for all its support, I know you cant do this with a Brownchurch.
Downsides of rear mounting are that after a long days drive the back of the tent (above the door)is normally covered in dust and needs a light brush off before opening. If you mount the tent at the front then this does not happen.
With the tent mounted at the front the rear of the rack is free for any other gear you are carrying - gas bottles etc and much easier to access from your rear ladder than climbing up the front and over the bonnet.
Finally you all like to sleep with your heads higher than your feet right ??? if you mount it at the front you climb up, sit in and lie down ( the rear suspension is normally higher than front - so you would normally lie feet to the front of the vehicle), if you mount it at the rear you climb in sit down turn around and then lie down...Id always get my feet on my girlfriends pillow !!!
If you do fit your Roof Tent to the front or rear on a Land Rover make sure you use extra support bars from rack to bulkhead/rear body as the guttering is not strong enough to support heavy weight on its own over a long period of time - its only rated for 75kg including rack weight - you have been warned !
Just a word on The Hannibal - its a good tent - things that are not so good - the Fly sheet is pants - and I think the sprung poles are a poor design. In hot african weather in dry season the Hannibal was bloody good - more ventilation and cooler than any other (same as Ezi Awn) with the fly off - after prolonged rain though the fly is not up to the job.
The Brownchurch is a better designed tent - not as cool in the heat as the Hannibal, but more suited to travelling through Europe in winter and a rainy season, the fly sheet mounting with the gavanised supports is much better than the Hannibal / Ezi Awn method and the build quality is better.
Hope this is of help, Cheers Grif.