Fortunately I have no experience of having to land with no gear, but I've seen the aftermath of a few on both tarmac and grass.
The first was on grass and was inadvertant (A v.senior training captain who said he was used to having an F/O remind him!) Luckily the grass was very smooth and the touch-down gentle. The damage was limited to the prop and the flaps.
The second was on tarmac and was intentional (I can't remeber the exact reason why). The touch down was again pretty smooth but the damage done was a lot greater.
At the end of the day I would always choose tarmac with a foam blanket or water sprayed on simply because of the guaranteed smooth surface which I feel is the most important factor.
Damage to the a/c is of very little importance as long as you get out ok, though if you do less damage to the machine, there is less force to be transmitted through to us.
Though if I had a choice between a smooth grass surface and tarmac I'd go for which ever had the better emergency cover and more importantly, which place had the best bar 'cos let's face it, you'll need a drink afterwards!