Check out my thread on on grapple pics, it might be useful, if not mildly entertaining :laughing:
There are advantages to two grapple arms with the main disadvantage being weight. bottom line is you can do everything with a 48" grapple that you can do with a wider grapple. The only difference is you can typically do more, faster with a wider grapple and 2 grapple arms. The times where the extra weight and wider size can be a disadvantage, are when you are right at the limit of your lift capacity, and when you are digging with the grapple. You will be able to dig better with a narrow grapple, due to the fact that you are applying more force to each tooth that is digging.
I have a 64" WR Long grapple and wouldn't trade it for the world. I am more than happy with the product and customer service. There has been a lot of talk about Wildkat lately so certainly check them out. Be sure to check the weights. Mine is right around 500lbs, I want to say 485 from memory but I have a bad memory
I was clearing some land a couple weeks ago and long story short, I got more done, faster with my wider 2 arm grapple. I was able to knock several trees down, cut the stumps off (after ripping them out of the ground) and I could then carry 2 stumps per trip with one stump in each arm. These were smaller stumps from about 12"dbh trees. It cut my trips in half and when your carrying them a good distance this makes a difference.
For the larger stumps, you can only carry one at a time anyways, just like with a smaller grapple. Many people tried to talk me into the 48" grapple but I'm glad I went with the larger one, having said that, I have to agree that a 48" one armed grapple can do everything a 64" two armed grapple can do, I just think I save time with the larger one, mostly because I do a lot of tree clearing/stump moving, and a lot of firewood work. With my 64" flat open bottomed grapple, i can stack a lot more wood on there than the 48" would allow. It really depends on your uses. Since you mentioned digging, I would lean towards the narrowing grapple.
As far as clam shell vs. open bottom like the one in the pic you posted, I would go with the open bottom. I've used both of them and much prefer the style I went with, the OBG 2 from WR Long (or similar)
Sorry for the long winded post, keep asking questions.