The Scoundrel is a stealthy, positionally-dependent melee class regardless of which tree you select.
The Scrapper tree revolves around a few, hard-hitting burst abilities while improving your stealth technique. Solo, expect to use sudden, overwhelming damage to remove a target from the field alongside a wide range of crowd control to maintain your positional advantage. In groups, be prepared to provide long-term, pre-fight crowd control, excellent add management, and plenty of damage to the primary target. The downsides being extended periods of time off-target (melee classes are particularly vulnerable to knockbacks, highly-mobile targets, and anything else that requires you to re-close the distance).
The Dirty Fighting tree focuses almost wholly on damage-over-time. I've not tried this tree, but I would imagine that DoT mechanics would yield steadier damage (even you're knocked away from the target, your bleeds keep ticking) but a slower build-up (Vital Shot, Wounding Shot, and Hemorrhaging Blast all require time to work; the "T" in "DoT"). My understanding is that you're still stealthy, still melee, and still positionally-dependent -- you just back-load your damage (Cold Blooded) vs. front-loading it.