I thought long and hard =長い事考えた
長い事しっかり考えた、と言う意味です。
I gave a lot of thought to it =すごく考えた
I thought long and hard about it, and this is my conclusion =長く考えた結果こうなりました
I really couldn't decide which one I liked better, but I went with that one.
この場合、次のような言い方ができます。
A: Why did you pick that one?
「なんでそれを選んだの?」
B: I had a hard time deciding, but I chose that one.
「決めるのに苦労したんだけど、それを選んだ」=「結構悩んだんだけど、それにした」
または次のようにも言えます。
ーI really couldn't decide which one I liked better, but I went with that one.
「どっちが良いか、なかなか選べなかったんだけどそれにした」
ご参考まで!