I've not yet got around to trying them with my character who isn't maxed trade yet, so I am not sure of exact percentages and what not yet. However, doesn't it make sense that the 'easy' tradeskill writ (aka the non rush order) one will yield less benefit in xp than doing a rush order?
If I'm not mistaken Domino said both types of writs would give the same xp percentage. If you want to level more efficiently with writs, then you do the rush writs, which involve fewer combines for the same % xp for completion.