Synthetic jackets can act as mid-layers, stand-alone jackets, or puffy parkas that fit over your shell in the coldest of temperatures. Unlike down fill, synthetic insulation will provide insulation even when it's wet. It's also not nearly as expensive. The type of synthetic jacket you choose depends on what you're going to use it for. For hanging out on belay or cruising town in the cold all you need is something basic that's nice and warm. If you're skiing, backpacking, or climbing and pushing yourself, then weight and features start to make a big difference.
Most synthetic-insulated jackets use a lightweight or medium-weight nylon shell with a water-resistant DWR coating. However, a puffy with fully waterproof fabric will be worth its weight in gold in you think you'll encounter wet weather.
How you use your jacket determines which fit is best for you. Choose a slim, athletic, or medium fit if you'll wear your down jacket as an insulating mid-layer, or opt for a loose fit if you plan to layer your down jacket over everything else.
A synthetic-insulated jacket with relatively little insulation makes an excellent choice as a versatile mid-layer when you're moving or your primary layer when you're just hanging out by the campfire. Opt for a jacket with more insulation if you're standing around while belaying an ice climb or you know you'll encounter brutally cold weather.