(2016-02-18, 9:21 am)PotbellyPig Wrote:(2016-02-17, 7:13 pm)zx573 Wrote: Have you checked どんな時どう使う 日本語表現文型辞典? I don't know sure what nuances are listed at the bottom of the description in Kanzen Master N1 so I can't say for sure if it's what you're looking for, but it has a lot of N1 grammar (and everything before) but it's been pretty good in my experience for everything I've looked up. べく and んがため（に） are both in there, too.
Does it go into detail on why you would use one over the other?
It doesn't compare and contrast the two grammar points, no. But the definitions seem pretty clear to me in the book I suggested.
べく
1) Someone tries to achieve goal they set. Formal expression, but also used in modern speech.
2) Clauses of requests, commands, or appeals do not follow.
んがため（に）
1) Person in question has proactive goal he truly wishes to accomplish. Formal written form.
2) Clauses containing requests, commands, or urgings do not follow.
From what I gather between these two, you could potentially use べく when speaking but wouldn't use んがため（に）, and that んがため（に） would be used when the person is really going for it where as the person being talked about it べく might not be as proactive, but are still trying to achieve their goals. It makes it a bit clearer compared to the explanations I see in the Shin Kanzen Master N1 Grammar book, anyway.
I pulled an example sentence for both points that seemed to represent the difference well:
べく
1) ラムさんを迎えるべく空港まで行ったが、会えなかった。
んがため（に）
2) 研究を完成させんがため、彼は昼夜寝ずにがんばった。
ラムさん did what (s)he wanted to do but didn't seem all that dedicated, whereas the other person did whatever it took to complete his research.
Edited: 2016-02-18, 2:32 pm