I love assualt cannons. Always have. From my first exposure to Citadel Miniatures with Space Crusade when you were allowed to take them on your power armoured marines, they have always been on of my favourite weapons. Plus, I'm not quite sure why, but they've always seemed "wolfy". I got a box of Wolf Guard Terminators for Christmas many moons ago and I still love the guy with the Assault Cannon. Now, there is a rumour going around t'internet that the assault cannon is better at penetrating AV14 than a Lascannon. Let's look at the maths...
P(Penetration) = P(Hit) x P(Pen) x Number of shots
= 2/3 x 1/6 x 1
P(Penetration) = P(Hit) x P(Rend) x P(Pen) x Number of shots
= 2/3 x 1/6 x x 1/3 x 4
So, it looks like an assault cannon is better. However, as they say, there are lies, damned lies and statistics. One of the few things I remember from stats at university is they are meaningless without some sort of context. Now, the Lascannon has one big adavantage - range. 48" will let you reach out and touch that enemy vehicle with little reprisal. If you've got a vehicle mounted one, you've even got an effective range of 54". The Assault Cannon has a listed range of just 24" but since every platform you can mount it on (namely Razorback, Terminator or Land Speeder) can move and fire it becomes an effective range of 30".
Now, the main issue is that neither of these guns are that good at reliably taking out AV14. However, Lascannons are brilliant at taking out AV11 and AV12 (ie most transports in the game) and Assault Cannons are brilliant at taking out infantry (and so-so at taking out those other transports). I take twin-linked versions of both weapons in my army and I try and remember what their main targets are. Only shoot at AV14 if there are no other targets available or if you're having a day where your dice are really hot.