I'm playing an alchemist in a steampunk campaign.
I'm going to take the explosive missile discovery for flavor reasons only.
It allows you to use your bombs with ammunitions or bullets (only onehanded firearms, but that what I would have taken anyway). I'm not going to do secondary attacks (not my role, I don't want to be a striker), so don't take those into account.
I'd like both options, but I slightly prefer the crossbow since it's make more sense to apply a poison to a bolt than to a bullet (I don't think the rules prevent to do that). The problem is that using a crossbow requires a normal ranged attack instead of a touch ranged attack (which may make it worse than simply trowing my bombs): so, is there any reason to use a crossbow in this case instead of a gun?
If your gm allows firearms, then the only real reasons to use a crossbow over a gun, is if you like it more, and cheaper ammo. And yes, using bombs with a crossbow will require you to hit normal ac instead of touch, thus resulting in your bombs missing more often.
Crossbows mean you're hitting normal AC instead of touch, so I wouldn't do it, as even if it does slightly more damage on a hit, you're going to be hitting a lot less.
Guns would be the better option, but the problem is that you would have to take a feat for proficiency, plus Rapid Reload so you don't have to spend a standard action reloading a one handed firearm.
If you like the crossbow for flavor, but prefer the gun mechanics, why don't you see if your DM will let you take a custom made clockwork crossbow that shoots arrows with the help of gunpowder (that you can then treat as if it was a one handed firearm as far as all the rules are concerned)? It would certainly be steampunk.