Just finished the game's story. Gunvolt's abilities are explained much, much better than in the first game. Copen, not so much, but when you figure out how his abilities work the guy is a beast.
Copen plays like Beck, Model A, and Model H all rolled into one. Inti basically said, "hey, let's take the dash-ramming an enemy thing and make it not suck" and made it your lock-on instead of your high score finisher. The boss weapons are just varying flavors of extra damage, which you'll need, since Copen's default abilities take a bit longer to whittle down the enemy than GV's (those water lasers, though). His special skill Shred Storm kinda sucks, but his defenses are near impenetrable. So long as you don't get careless with your air-dashing Copen can pretty much prevade into eternity, and if you do find yourself in trouble his heal skill regenerates relatively quickly.
GV has an extra "Clip" gear which decides his tag count independent of what Bolt type he's using. Generally speaking, lower tag count means more attack strength and vice-versa. Most gears have an EXP gauge that fills as you clear levels with them, full gauge gets you an upgraded version. It's a welcome change compared to the first game requiring you to synth two of the same gear together, however, the upgraded version is NOT usable as synth material at all, so you actually need to make sure to not use gear that you're intending to synth into something else entirely.
Challenges are now always active after your first clear of a given stage, which makes things far more convenient.
Racking up Kudos is MUCH easier. The game by default gives you three hits until you lose it (as opposed to one in the original). You can change it to one hit for a higher score multiplier, or NEVER lose it in exchange for a lower one. Combine that with one of GV's eye upgrades that removes the level checkpoints, and yeah, crazy (I assume Copen has an equivalent armor upgrade but I've not unlocked them all).
No more chaos emeralds jewels to collect, and no requiring the use of underpowered gear, the extra ending is unlocked simply by replaying the final stage after you already cleared it as both characters. Grab a tissue box, though.