This will depend on some key deciding factors. Both offer great insulation, that have some different benefits. If you primarily want to be warm, and will use the jacket in dry or snowy conditions, then down insulation is a great option. If you want to be warm, and will often use the jacket in wet conditions, then a synthetic insulation will be better suited to your needs. If you need the jacket to be particularly lightweight and packable, then down has the upper hand. A down jacket of the same size and volume, will pack smaller than the synthetic version. There will always be discussion as to which is better, but it really depends on your priorities and the weather conditions you are most likely to encounter.
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