How should I read 早々？ As そうそう or はやばや？

Question

How do you read 早々　in these sentences?

I learned there are two readings: そうそう、and はやばや。

How do they differ, if at all?

早々にごへんじいただきありがとうございます。 Thank you for your prompt reply.

新年早々、借金を催促されるなんてひどすぎる。 It’s too cruel to demand money right after the start of the new year.

Answer 1

There is a good explanation about the two readings at 例文.jp.

To sum it up:

早々{はやばや}

• Nowadays generally used as an adverb with the particle と
• Means "early", "immediately", "quickly"

金曜日は早々{はやばや}と仕事を切り上げ、教室へ直行するのが常であった。
On Fridays, it was usual for me to finish my work quickly and go straight to the classroom.
_{Source: 例文.jp}

早々{そうそう}

• Used adverbially with the particle に; same meaning as 早々{はやばや}と (but they aren't interchangeable grammatically)
• Can be used as a suffix to nouns with the meaning "right after ..." with particles から, に or no particle at all
• This is apparently also used in formal letters. Please refer to these answers: What's the appropriate 挨拶 for e-mails to a friend and for e-mails to customer service?

Based on this information, 早々 should be read そうそう in both your sentences. In the first one the 早々{そうそう} is used with the particle に. In the second sentence 早々{そうそう} is a suffix to 新年 and 「新年[早々]{そうそう}」 means "right after the start of the new year" like it's translated.

Answer 2

On WWWJDIC, there are two entries.

早々(P); 早早 【そうそう】 (n-suf,n-adv) (1) as soon as...; just after...; immediately after...; (adv) (2) (usu. as 早々に) hurriedly; in haste; quickly; promptly; early; (P)

早々(P); 早早 【はやばや】 (adv,adv-to) early; quickly; promptly; (P)

As you can see from the entries above, the parts of speech for each of these two readings are different.

Based on this part of speech info, this seems right to me:

早々【そうそう】にごへんじいただきありがとうございます。 Thank you for your prompt reply.

新年早々【しんねん・そうそう】、借金を催促されるなんてひどすぎる。 It’s too cruel to demand money right after the start of the new year.

This is because these both seem to be nounish usages.