I have both and appreciate both. My 20 deg down quilt is pretty ideal for dry, semi-cold conditions. But when you're looking at sustained cold wet conditions a synthetic bag is going to offer a much better comfort margin. The idea being you can actually climb inside a synthetic bag in wet clothes and expect to dry out. Both a quilt and down insulation would probably let you down in that situation. As for versatility probably nothing beats a 20 degree synthetic bag, for lightweight packability nothing beats a down quilt. Lot's to read on here as this debate is endless.
If you really can't decide and don't mind foregoing a mummy bag, the enlightened equipment convert with synthetic insulation is a pretty cool option that's not too expensive and is about 6-8 oz lighter than an equivalent synthetic bag. It can also be had in down. It's a zippered quilt you can use either way, defeats the purpose of a quilt in my mind but is a useful way to have both if you truly are stuck on the fence.