I have seen a lot of debate lately over which big blue storage vender puts out a better box, IBM or EMC. I am definitely a huge EMC fan and felt compelled to weigh in on my opinion in the matter from a technical standpoint. I have taken Scalability, Performance, Availability, Management, and Interoperability.
IBM v7000 is limited to 240 drives with a non-clustered v7000. Clustered v7000 carry many caveats and limitations. Storage is also captive in each controller.
EMC VNX5400 is the smallest model in the VNX family and scales to 250 drives, and can be upgraded data-in-place to VNX8000 to support up to 1,500 drives with no limitations.
IBM v7000 only has 16GB of SP Cache: 8GB of cache per SP
EMC VNX5400 has 32GB of cache, an additional 1TB of FAST Cache can be added as L2 Cache. VNX8000 can support 4.2TB of L2 Cache.
v7000 snaps have an impact to array performance
EMC VNX5400 has up to 256 snaps per volume, that space efficient, RoW.
IBM v7000 is susceptible to catastrophic failures and silent data corruption. CERN did a study on this that is written up here http://storagemojo.com/2007/09/19/cerns-data-corruption-research. v7000 does not have data integrity features: No CRC or T10DIF.
EMC VNX incorporates CRC and proprietary Sniffer technologies to ensure integrity of data on disk. For example, we format drives with 528Kb, instead of traditional 520Kb. The extra 8Kb is used to store a checksum for a CRC check.
EMC Recoverpoint provides very granular data recovery with low RPO and RTO for remote replication. Recoverpoint also provides up to 15X bandwidth reduction with deduplication and compression.
Management is good with both platforms, however; FAST or Fully Automated Storage Tiering is far superior in the VNX. As an admin, you don’t have to worry about managing or optimizing storage for cost or performance, the array makes those decisions and migrates data for you for most optimized data placement on the correct storage tier (SSD,SAS, NLSAS). This is based on how your applications access your data. Its dynamic, just like your applications and data over time.
EMC VNX5400 also has introduced Multi-Raid Caching in which all reads and writes going to the VNX are put into one global pool of cache between both storage processors.
IBM has great interoperability testing IMO, but are typically late with support of new software features. As an example, with the announcement of vSphere 5.x, VMware announced that EMC will support every new feature day 1, IBM will not. For example, IBM does not support Thin Stun and Space reclamation primitives for VAAI, EMC VNX does.