E3 xeon are moreless an i7 with ECC support running at lower clock, thus being more stable in very long executions, another plus.The Xeon E3 isn't any faster than the i7 used in the current top-end iMac... Only real plus here is ECC memory, the rest is marketing.
An Xeon iMac wont be faster than an i7 iMac but will be much more stable running long task, (also Apple should improve its thermals, current iMac isnt quite good dealing with heat).