Difference between 怪しい and 疑わしい

Question

What's the difference between "怪しい" and "疑わしい"?

The two entries in weblio are similar (怪しい versus 疑わしい).

Apparently, they're similar enough that dictionary.goo.ne.jp has a guide for distinguishing the two words (along with two other words), but it's in Japanese and Google Translate doesn't do a good job of translating it.

As background, it's being used to describe an ad that had a stated requirement, but where it's suspected the creator of the ad had an ulterior motive in mind. "疑わしいアド" was used in my original version of an online post, whereas "怪しい求人" was suggested by a native speaker of Japanese.

I've just discovered that Wiktionary has some info on distinguishing the terms (order reversed for ease of reading):

怪あやしい (ayashii) implies something for which the true nature isn't known, and this uncertainty is a bad thing.

and

疑わしい implies something for which the true nature or quality isn't known, but that may be good or may be bad.

which is at least in English, but still seems a little confusing to someone who is somewhat unfamiliar with Japanese culture.

Answer 1

An difference between them is that [怪]{あや}しい is near to spoken language and [疑]{うたが}わしい is near to written language in my feeling. Of cource both 怪しい and 疑わしい can be used in both spoken / written language though.

Examples

Here are some examples of non-swappable case.

Example 1

In ordinary conversation,

OK: あの男の人、[怪]{あや}しいよね。 (That man looks suspicious, doesn't he?)
NG: あの男の人、[疑]{うたが}わしいよね。

In above case, 怪しい sounds natural but 疑わしい does not.

Example 2

In educational document,

OK: この[数値]{すうち}は[出典]{しゅってん}が[示]{しめ}されておらず、[疑]{うたが}わしい。
(This value is doubtful / suspicious that does not indicate the source.)

NG: この[数値]{すうち}は[出典]{しゅってん}が[示]{しめ}されておらず、[怪]{あや}しい。

In polite document or speaking, 怪しい is not suitable.

Answer 2

according to the link you posted (http://dictionary.goo.ne.jp/leaf/thsrs/3345/m0u/) 疑わしい only means that the information is uncertain/doubtful. 怪しい means that the uncertainty/doubt is a bad thing.

If i had to put it in simple terms 疑わしい would be 'doubtful', while 怪しい would be 'suspicious'.

As background, it's being used to describe an ad that had a stated requirement, but where it's suspected the creator of the ad had an ulterior motive in mind. "疑わしいアド" was used in my original version of an online post, whereas "怪しい求人" was suggested by a native speaker of Japanese.

in this case 疑わしいアド would suggest an advertisement of uncertain validity (without the negative connotations, e.g. posted as a prank), while 怪しい求人 would mean suspicious job offer (e.g. some mafia hunting for prostitutes, disguising it as a job offer)