I'd go with saikei because the focus seems to be more on the landscape than on the trees themselves. There is not a lot of definition in the trees (they are more silhouettes than one would strive for in bonsai), but there is more than usual care placed in the layout of the ground and rocks. All that said, I suppose a saikei would be a form of group planting so I guess the real questions in my mind is why it actually matters what it is, and why it can't be both? I quite like it. I think it could go into a tray that is another 1/3 to half wider again to give it a feeling of space, and I think that would push it even more towards a saikei as it would take the focus even more off the trees which are quite heavy visually.
Well, I'll use my anus ( ) and say I reckon it's a group planting. reasons being, lack of 'structure' to the rocks, lack of ground cover or accent plantings and the lack of any 'story' to the setting.
Are there 5 or 6 trees in that tray? Not that it matters, anyhoos.
Graeme I will forever defend your right
to disagree with my opinion.