There are many (loan) words which have both katakana version and pure Japanese version.
For example:
report = レポート or ほうこく (報告)
project = プロジェクト or じぎょう (事業)
task = タスク or かだい(課題)
I read What causes loan words to displace native words for existing concepts? But that does not clarify which one should I prefer over other?
Should I use katakana version for casual conversation and the native Japanese word for formal conversation?
On business, maney Japanese use katakana-English and Pure Japanese (came from Chinese), and almost of them are believed interchageable but always has slight difference each other. I can't reffer all of them here, but the policy is simple "Use Japanese version as many as possible."
You must has realized that most of all Katakana-English are nouns.
レポート = 報告、報告書
レポートする = 報告する
Each Katakana-English does not always have the same meaning of Japanese version.
プロジェクトはプロジェクトで、これを事業とは言わない。
事業 is used in more broad sense than プロジェクト。事業 is devided into many projects and rutine operations.
タスクと課題も異なる。 そもそもほとんどの日本人は、task と job と workの違いを知らない（気にしてない）。
それに、
「課題と問題は同じではない」などとよく言われる。問題は「進行を妨げる諸々{もろもろ}」で、課題は「問題を乗り越えて先にすすむためのひとまとまりの仕事」
タスクチームは、上述の課題解決のために設けられた組織（チーム）。
Once you learn Kanji-versions, you must memorize the original English terms respectively. When Japanese use Katakana-version, you always aware of the original English term and you can understand the difference among the Katakana-version, Kanji-verson and the original English term.
Hope this helps.
I read an article from japantoday.com Top 5 myths about learning japanese. From the article:
#2. You need to use native Japanese words instead of borrowed foreign words
As we all know, Japanese has a lot of borrowed words in it that range from perfectly understandable to rage-inducingly absurd.
But like it or not, those borrowed words are part of the Japanese language. Saying you don’t like them is the same as saying you don’t like the English words “admiral” (Arabic), “ketchup” (Chinese), or “jungle” (Hindi).
And yet so many students insist on using the native Japanese equivalents of words when there’s already a perfectly-fine borrowed word ready to use. They say to for “door” (instead of “doa”), “daidaiiro” for “orange” (instead of “orenji”), or “taku” for “table” (instead of “teeburu”). They think it sounds more “pure,” but really they just end up sounding silly or outright wrong.
Unless you think saying “Let’s eat seaweed-wrapped-rice-with-raw-fish” sounds better than “Let’s east sushi,” please don’t do this.
One of my Japanese friends said they always use クラス instead of じゅぎょう.
I also read some facebook posts suggesting that these loan words are perfectly ok to use.
Conclusion
Loan words are now part of the Japanese language. We don't have to choose native Japanese words over loan words.
