I looked these words and means surely. Usually I see きっと, but is usual to use さぞ?
It's ok use both?
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Both きっと and さぞ are adverbs of epistemic modality (which means they express a type of uncertainty), but they're different in a number of ways:
Level of certainty. きっと is more certain than さぞ.
Frequency. きっと is significantly more common than さぞ.
Register. きっと is normal in conversation.
I've been told by more than one native speaker that さぞ sounds rather old-fashioned.
Pragmatic restrictions. This refers to which contexts a word is appropriate in.
さぞ is used under a narrower set of circumstances than きっと. When the speaker uses さぞ, they're expressing sympathy with the hearer or the sentence subject. It's somewhat subjective in nature. If this requirement isn't met, using さぞ is inappropriate.
きっと doesn't have this requirement, so it can be used more generally.
Modal harmony. This refers to co-occurrence with other expressions of modality.
The adverb さぞ is very likely to co-occur with だろう (or でしょう, etc.) as in your example.
Although きっと also co-occurs with だろう, it appears without it more often than さぞ does, and the greater degree of certainty means it's also possible for きっと to harmonize with relatively certain expressions like にちがいない.
Although both adverbs are possible in your example, as you can see they're not quite the same. By the way, さぞ is usually written in kana.
For more information about modality from a linguistics perspective, I recommend Heiko Narrog's Modality in Japanese (2009). Modal adverbs such as きっと and さぞ are covered on pages 108-110.