In Japanese what would be the correct way to say something along the lines of this expression which would be common in English.
For example if you try to say some people like dogs and other people like cats.
What immediately comes to mind for me would be 人は犬が好き、けどほか人は猫が好き。But...though the second part seems OK to me the first part feels...too absolute.
某人は犬が好き？
Basically I'm trying to wrap my head around more advanced comparisons.
The typical translation would be 「犬が好きな人もいるし，猫が好きな人もいる」 ． (Lit. There are some people who like dogs and there are others who like cats.)
One possible translation along your attempt is 「いくらか[or 幾人か，etc.]の人は犬が好きで，他の人は猫が好きだ」. This makes sense and is grammatically acceptable, but it is awkward: Although I've encountered several books (translated to Japanese) which use this kind of translation, it reminds me of those word-for-word translations made by junior-high students.
某人 isn't listed in any of my dictionaries, but would be understood by most Japanese speakers as a substitute for [某氏]{ぼうし}. 某氏 is used to denote a specific but anonymous person (of either unknown or hidden identity), so 「某氏は犬が好き」 is a correct expression but bears a different meaning --- "A certain person likes dogs".
(Edit: 某人 has several examples as [某人]{あるひと}(google search with site:aogora.gr.jp).)
