While not as lightweight or packable as down, synthetic insulation is water-resistant and still insulates when wet, making it ideal for wet environments. Plus, it's considerably less expensive than down.
Mummy-shaped bags taper towards your feet to reduce volume and hold warmth closer to your body. These bags can feel restrictive to some. A semi-rectangular bag can offer more room to move.
If you're facing wet conditions, look for a bag with water-resistant properties. DWR coatings on the outer fabric repel light moisture to protect against mists, drizzles, and snow.