I think Katie addressed this pretty well recently. I'll try and do the same.
An assault cannon is about 1/3 more effective than a lascannon against AV14 (in terms of penetrating hits; it's probably better than that at glancing).
So, all well and good. Do you get to fire that assault cannon as many times as the lascannon? The lascannon has double the asscan's range, and furthermore firing from that range means less things are firing back, so you're less likely to get damaged.
Those are two good reasons to take the lascannon, unless (like BA) you have a lot of transports coming in quickly, you want to get closer anyway, and/or you just happen to be Fast.