In truth both are good - I chose the 'Devil' 600 over the Monty as it has excellent space /porch - and is inherentally stable if pitched correctly You wouldn't go wrong with either! Monty's sold out last year so there's few if any 05 models at keen prices - D400/600/900's are available at cracking prices e.g D600's sub £200 on the net.
I recently had my mind set on buying the Diablo as a tent upgrade this year, that was until I went to a tent exhibition where I saw them all erected. This is when I noticed the rather small bedrooms which also had sloping roofs. The Montana 6 was also on display and we were immediately drawn to it. There is no comparison between the two and if you need the space, I would look no further than the Monty. The Monty is also a much better quality tent and has an HH rating of 5000, compared to the Diablos 3000, offering much better weather protection. The Monty 6 is a more expensive tent, but is far superior to the Diablo and is much easier to erect.
We've got a Diablo 600 last year and loved it. The only problem was it took a bit too long to pitch to take away for weekends (about 1hr)
So we have just bought a Montana 4 for weekends. Not been away yet but much quicker to pitch from our practice run in the garden.
Initial thoughts are.
Diablo- fantastic space, great porch for storing wet shoes coats, chairs etc, double skined throughout so cosy in the living area. Bedrooms are apart so don't disturb kids as easily when niping to the loo at 3am. Bedrooms do slope but not a problem if only 2 people in a pod.
Montana - much quicker & simpler to pitch, still a good sized living area, porch could hold shoes but nothing else. Full height living are but don't really see any advantage to this unless a) you like trampolining on the airbed, b) you want a double height airbed c) you prefer campbeds
If you are going away for more than a two or three nights I think you will be happy with either tent.
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We couldn't make our mind up between these two either, went to a yeoman display, and went for the montana 6, mostly because of the height all the way through, and it did seem easier to put up.
we have a diablo 600 which we used before we became caravanners ( we are going to sell it if you are interested) my partner often put it up on his own with a bit of help from our 8 year old son .( i was busy feeding the baby - honest!!!) we found it a brilliant tent with loads of living space and the porch area was fantastic for the puschair ,toys ,wellies and everything .as said before the bedrooms are not close so there is more privacy - we camped on the isle of wight with it and it withstood dreadful winds and torrential rain - many other tents caved in and my sister left with her trailer tent ! we used EHU with it and camped all seasons with a little heater because the living space and built in inner just made it so warm and cosy (and no bugs could get in ) so we liked it i do not know the montana as we never had one so i cannot comment on it
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.
Topic: Diablo 600 or Outwell Montana 6???
You wouldn't go wrong with either! Monty's sold out last year so there's few if any 05 models at keen prices - D400/600/900's are available at cracking prices e.g D600's sub £200 on the net. 
