There are many styles of grapples. I would not choose a brush grapple to pick rocks. As you can see the L-style that Xfaxman uses works great - so does the rock & root grapple I use. Different strokes - etc, etc. JMHO - but, its not so much the exact type of grapple that will make a good rock grapple - its more the skill and experience of the operator.
Do not forget - the purchase of the grapple is only part of the overall cost. You must also get hydraulic power to the grapple one way or the other.
What is the best grapple for you - TomIre - what fits your tractor and pocket book best. Remember - you are talking about the purchase of an implement that with proper use and maintenance should be expected to last a lifetime.
Example - my grapple that I'm talking about in post #7 & #9. The WR Long hydraulic over electric activation system - $1195 The Land Pride SGC1560 grapple - #3175 This is the price of the equipment plus installation.