Mirrorview/A or Mirrorview/S

I would say it has to do a lot with how fast you want to get the migrations done, how many LUNs you want to migrate at once, and wether you are willing to accept any potential performance impact on the production arrays.

If you have time to migrate things some at a time, I would tend to go with MirrorView/A. I'm sure there will be other opinions on this, but with /A you can migrate up to 50 LUNs at a time with no major impact on production performance. You can migrate the LUNs for a few servers at a time, shut down the servers, make sure a final synch is done, then point your servers to the target LUNs.

The best part about this method (in my mind) is that there is potentially a lot less impact on prod performance.

That being said, we don't use /S in our environment, so there may be some way of configuring it to meet your needs more effectively.

I used Mirrorview/S to migrate approximately 10TB of data off CX700s to CX3s. It worked very well for me. Be careful how many initial sync jobs you run simultaneously and the rate that you run them at, as this will have an impact on array performance. But, once it is sync'd up, assuming the destination array and disk layout is adequate, you shouldn't notice a negative impact to performance for your writes. For my migrations, I did the initial sync for 1-4 luns at medium or low speed. I migrated a couple hosts at a time over the period of a few weeks. I sync'd up data during the week and did the cutover on the weekend.

I have no experience with Mirrorview/A (nor the license key), so the choice was easy for me - use S. Since you have experience with A, that may be a better choice for you, but either should do the job.

Good luck.