Xeon all the way, especially if you get the non k version of the i5. Xeon has 8 logical cores mean while i5's have only 4. The clockspeed is almost the same and since both are non overclockable. Xeon is more stable and better quality, has bigger L3 cache, and faster too if i remember right.
All the points except the sligh clock speed diff is in the favour of xeon.