Mathemarically, the grandstaves are better against 4+ save and above. They are the same against 3+ and then swords+staves are better against 2+ and below. Given the rarity of 2+ and even 3+ saves, the grandstaves are better on paper.
However many feel more secure in having rend as this makes the attacks of the sword+stave more reliable despite being weaker on average.
Against rerolling saves (e.g. Kurnoth hunters), the rend is also more viable, and the swords+staves do more damage against 5+ rerolling save or better.
When taking a unit of ten evocators, I would equip 5 with grandstaves to fight in two ranks.