Either solution will work and will work well but there is a lack in real comparison of the differences. An organisation will choose one over the other not only on performance but costs and other operational considerations.
The Isilon used 108 4TB drives running in a 3 node configuration that took up 12U of rack space.
The IBM used 348 2TB drives running in 6 trays that took up 24U of rack space.
But the size of drives really has no impact on throughput performance where as number of drives does. If the number of drives in the Isilon is tripled the performance is 2.4GB/s compared to IBM's 5.4GB/s which is closer but still gives the advantage to the IBM solution.
But Isilon does have have higher performing nodes that would narrow the gap and the performance difference is likely to be relatively small between the two solutions.
So the one would want consider things like the IBM solution is using InfiniBand where Isilon is using 10GbE. IBM uses its proprietary GPFS protocol while Isilon uses standard protocols like NFS, SMB, and HDFS. Again neither is necessarily better or more right than the other but each may be more suitable for specifics situations.
Personally, the raw single stream performance of GPFS is impressive but not always needed where as the linear scale out and ease of use with Isilon is impressive and probably more useful.