The SAM4E16E is a hugely powerful chip indeed - (copied from other thread)...
A number of reasons;
- integral FPU
- largest SRAM & flash of all the ARM MCUs
- highest number of timers
However, it still isn't readily available - Mouser are showing a 20 week lead time with a price of around £6-7 in QTYs of 100s. Certainly my application as it is now doesn't require it, and the SAM3A4C is looking a very good bet indeed.