Mr Pig said:
I'm sure both types of frame will be more than strong enough, what I was thinking of is the more compliant ride of steel.
I have a Rock Lobster from Merlin Cycles (Leyland Lancs, not the US Merlin!). Its frame is made of 853 steel.
I bought my MTB in 2001 and it arrived 2 days after the great Foot and Mouth epidemic started so I wasn't able to ride it off-road for many months...
Merlin Cycles are selling an updated version of my bike and it looks good value -
link.
'Compliant ride of steel'? I'm sure that I might trigger off the old debate again, but as far as I'm concerned my steel Basso isn't any more comfortable than my aluminium Cannondale on the road. I can make the Basso unbearable to ride (on rough Yorkshire roads) by pumping the tyres up too hard. I can make the Cannondale comfortable enough by running lower pressures.
As for off-road riding on the Rock Lobster... I ride great big 2.3 inch knobbly tyres and choose tyre pressures that are low enough to be comfortable and high enough to make
snakebites a rarity even though I'm pretty heavy. I also have nice plush Marzocchi suspension forks and a USE suspension post so comfort isn't an issue for me.
The amount of 'compliance' you'll get from (suitably set up) tyres and suspension will greatly overshadow that of a frame of any material.
I don't fancy the catastrophic failure mode of carbon-fibre for off-road use so I wouldn't choose that material. I always fancied titanium but it was more expensive than steel. If you can afford it, why not go for Ti over steel since it has even better properties? Aluminium has to be the best-value option and it is lighter than steel, but I cracked my Al Bianchi road-frame so that rather put me off Al for off-road, even though I'm sure that most people have no problem with it. It'd just be worrying hearing rocks bouncing off the frame tubes on gnarly descents if you didn't trust the material

!
PS In summary - if I'd had the cash I'd have chosen Ti but I didn't and 853 seemed like a good compromise - I've not regretted it. My MTB is a bit on the heavy side but it is tough and reliable and I've enjoyed riding it.