An alternative to the 85/1.8 prime, the 100/2 is a very good portrait lens for full frame cameras. It's a bit of a toss up whether you prefer the look that a 100mm lens gives vs an 85. The 100/2 will give slighly more blurring to distant backgrounds and a slightly "flatter" perspective, but the 85 will give you a slightly shallower depth of field wide open. There's no clear cut "best" lens here and you can't go far wrong with either one. The 85/1.8 might be a touch sharper wide open, plus if you're shooting digital with an APS-C sensor and 85mm lens is already getting a bit long for portrait work and so may be preferred over the 100mm. The 85/1.8 is also slightly cheaper.