A slur, which is usually used to refer to the effect on a brass instrument, is different than a glissando, or a portamento, or even a pitch bend. Brass instruments are more or less "quantized" in that musical tone is reinforced only at multiples of the resonant length of the tube. So a realistic slur is actually not a "smooth" transition from one note to another, but an asymmetric slide from one resonant condition to the next. Although you can indicate a slur in notation, the notation does not produce the slur, the player does. A slur is also not a particularly musical adornment, it is more like a brass players kludge. Unless you are trying to simulate a brass instrument, a more symmetric transition may be desirable, and where control over the "shape" of the transition, pitch bend may be the most flexible control. But as in the real world, simply writing notation to indicate the slur, will not create a controlled slur. The most flexible control may be with a physical controller on a particular synth.