I chose bolt caster first, it wrecks mobs and can take down (most) yellow bars with one special attack, or two max. As well, it has tracking so if it's aimed in their general direction it'll usually hit them. Really good at wrecking witches even though their shields resists it's damage. Second, I'd pick solar. Especially for taken adds (captains, Knights, etc.). The key to me getting my black spindle is I had a friend with the solar exotic and he wrecked all captains and knights before they had a chance to lay down fire or blind. As well, this thing will one hit yellow bars like crazy and does an uppercut so can pull down witches too. Lastly, I'd choose bolt caster because it wrecks mobs up close and far away and its arc reaches for such a long distance with no damage fall off, has tracking, and wrecks in crucible. Worrying someone is gonna shotgun you or swing their sword on you in crucible? Just hit them from a mile away with your special. It's super versatile, definitely worth getting.