Other players are probably the best resource to answer such questions. As an ALD supporter yea...your interdiction rates will go up no matter which power you join. And yes, the threat of being attacked just for being aligned to a given power while in "enemy" space (or even in your aligned systems) is real as supporter NPCs from enemy factions attempt to hunt you down. Also, shooting an "enemy" (an NPC or player aligned to a Federation, Alliance or Independent power) who decided to shoot at you unprovoked while in THEIR space will get you a fine and a Wanted tag. So essentially you have to run from fights in enemy systems. This can make participating in some CGs awkward, though they are usually in neutral space.
The Pros of course are access to different weapons and modules (note: I did not say "better" weapons/modules for a reason) and in the case of ALD, a big bonus in RES hunting payouts. If bounty hunting is what you enjoy doing, a 20%/30%/40%/100% bonus to your check is a pretty huge deal. In those scenarios, people are known to make upwards of $30 million/hour RES hunting with a Rank 5 bounty bonus.
Note: You can get the same bounty bonuses from Winters if you want to pledge a Federation power with the same bonuses. With the addition of the Assault Ship and Gunship, there is no real reason to go Imperial unless you prefer those powers. Before it was about getting access to the Clipper and Courier but IMO the Assault Ship and Gunship are far superior ships worthy or grinding rank to get access to them. Plus, if you can raise the coin quickly, you can get a Clipper *without rank* for $14.7 mill.
$0.02