Pro mid-range zooms generally have a fixed aperture, usually ƒ/2.8 or ƒ/4. The variable-aperture zooms in this focal-length range are commonly consumer kit lenses—good, but not pro-level performers.
The fast zooms—ƒ/2.8 and ƒ/2—are better suited for dim-light work than slower ƒ/4 lenses. The fast lenses can also produce more limited depth of field to throw the background really out of focus and concentrate the viewer’s attention on a specific portion of the subject. Generally, the bokeh—background and foreground blur—is more pleasant with the faster lenses, too.
On the other hand, the ƒ/4 zooms are compact, making them easier to carry, and the slower zooms cost less. If you don’t need the benefits of the faster lens like low-light capability and extremely limited depth of field, you’ll probably be better off with the slower option. And, by and large, the ƒ/4 zooms are sharper at ƒ/4 than the faster zooms are at ƒ/2.8.
Note that the effective angle of view isn’t the only thing that changes when you switch camera sensor sizes. The depth of field produced by a given aperture also changes. A 12mm ƒ/2.8 lens on an MFT camera provides the field of view and depth of field of a 16mm ƒ/4 lens on an APS-C camera and a 24mm ƒ/5.6 on a full-frame camera when focused at the same distance, providing the same framing and perspective. That’s why you should go with full frame if you like those extremely limited depth-of-field effects. To get the depth of field a 24mm lens provides at ƒ/2.8 on a full-frame camera with equivalent framing, you’d need a 12mm ƒ/1.4 lens on an MFT camera—and there aren’t any of those. Of course, if extremely limited depth of field isn’t important in your work, this won’t be a consideration.
Zoom lenses offer several advantages over a set of prime lenses covering the same range of focal lengths. First, obviously, are cost and bulk. It costs less to buy one zoom than several primes, and it’s easier to carry the one zoom around than a bag full of primes. The zoom also allows you to quickly reframe with the twist of a wrist, rather than having to physically remove one lens and attach another each time you want to frame tighter or more loosely. And, of course, minimizing the number of lens changes helps keep your sensor free of dust.