Harlequin Intrigue vs. Harlequin Romantic Suspense
As their senior editors, Denise Zaza (Intrigue) and Patience Bloom (HRS) tell prospective writers in their linked Q&A chat, the two lines are definitely not interchangeable.
In a nutshell, I'll summarize what they're saying, adding in some examples from my own experiences:
1. Intrigue is a bit shorter (65-70K), more focused on the criminal investigation/procedural aspect of crime solving, and extremely "hero-centric." The more alpha the male is, the better. The romance element is less predominant than the intrigue focus, sensuality may vary but must be fully integrated with the crime plot, and love scenes receive less emphasis, if they're in the book at all. (My most recent Intrigue, Relentless Protector, involves a very suspenseful hunt for the heroine's missing five-year-old, and I can tell you, she was *not* in the mood, though there is definitely a developing romance with the hunky ex-Army Ranger hero whose wartime experience contributed to her late husband's death.) One or both of the protagonists is usually (though not always) involved in a higher-risk career, i.e. police, special ops/other military, private investigator. And cowboy heroes are very popular, though there are certainly other (alpha, Alpha, ALPHA!) types.
Passion to Protect, is the story of a wounded wilderness--hotshot--firefighter who's been carrying a torch for his first love, who married the wrong man. Battered but determined, she returns home, but is forced to rely on the hero to help her save her family from a devastating fire...and an even greater threat.) Can you see how these two stories differ?
A lot of these differences will ultimately boil down to the individual author's voice and storytelling emphasis. I started out writing Intrigues, which I still love, but my stories have gravitated more toward more emotional/family-oriented suspense, which has landed me squarely in Harlequin Romantic Suspense territory (which, I'll admit, is my favorite line to read. Don't tell! ;) By reading many examples from each line, you'll get a better feel for what the editors are looking for. And both are actively acquiring new authors, so best of luck to you!