Forge Guns do -10% to shields and +10% to armor, point your forge gun in-game at something and it will show a 90% efficiency at shields.

http://web.ccpgamescdn.com/dust/news.control/64862/1/damageprofiles.jpg

Not sure what you mean by vulnerable.

Play style is very different for the both of them, it comes down to two things, handling and acceleration. Total EHP doesn't really make a huge difference, that said the incubus will survive hits that a python couldn't dream of taking, but thy are more likely to get hit on the first place IMO.

Anyway, the big difference is in the handling and acceleration, the two handle very differently. The incubus handles very smoothly, it seems to glide everywhere and you generally want to start whatever manoeuvres your doing a second or two earlier than you would in the python to give the dropship time to start moving, the python on the other hand is very light, you can quite literally throw it around and it will do stuff the incubus is too heavy to do, you can quite literarily leave a turn to the last second and you should be fine.

The other difference is acceleration, this is the most important difference, the incubus feels like it's always at the edge of it's power, it's acceleration has a feel of "just enough", add on the movement penalty from armour plates and it quite frankly feels slow and heavy. The python on the other hand is a very different beast, it constantly has a feel of "GIVE ME MORE!!!!!" power wise and will just fly compared to the incubus, the thing is fast, an it always feels like there's more power in reserve should you need it, this contributes hugely to how light and manuevarable the python feels.

In effect these differences mean that the python is the king of air to air and it's pretty respectable as an CAS aircraft. The incubus however is just dismal at air to air, air to air combat at it's most basic in dust ends up as a thumb war, he who has the height advantage wins, they have the largest target possible (the dropships spine) while being a very hard target to hit without losing more height (due to the downward facing nature of the turrets).

The incubus is pretty decent at air to ground, the smoother handling and higher EHP make it better suited for that role IMO.

As for turrets, while blasters wreck infantry an shields their aiming is dismal and they're terrible at A2A combat. Missiles are the order of the day because they can be used against everything effectively and have enough direct damage to OHK nearly every infantry fit on the field, with the only exceptions being heavies and super tanked caldari logi's.

Use the Incubus it can soak up dmg, and swarms don't mean jack to a good pilot who can keep movement up and manage the cooldowns on his modules.

Its just like with tanks atm, armor has better hardeners and reps and at about 700k a pop you want survivability to be able to get those 3 or 4 kills you'll manage while half the enemy team tries to shoot you down for the bonus points and isk.

Whats the best way to fit an incubus for survival?