I've seen quite a lot of people advocating the lascannon/plasma Razorback recently and I was personally curious to see if it outperformed the assault cannon variant for close-in firepower. Obviously the twin-linked lascannon Razorback is the go-to for long-range punch, which is why I play one to supplement my other ranged firepower - Vindicators and meltas fill the close-range firepower well enough.
Using my mathhammer spreadsheet I did a binomial distribution to approximate the probability of getting at least one destroyed or immobilised result with each razorback turret, at each range bracket that's relevant: 1-12" for plasma rapid fire, 13-24" for plasma and assault cannons, and 25-48" for lascannons.
Past 24", obviously, the twin-linked lascannon is best.
Inside 24" the assault cannons start showing off how awesome they are, but the lascannon/plasma is competitive as well. You'll notice that the twin-linked lascannon is actually the worst against AV 11 and 12, which are the two most commonly encountered values, and the twin-linked assault cannon is very close in terms of Land Raider-killing ability.
Things get really interesting inside 12". The lascannon/plasma turret is awesome at this range because the plasma is rapid-firing, but the catch is that if your razorback moved at all, you have to choose between firing the Lascannon or the plasma gun (per the Space Marine FAQ, they are separate weapons). That means that it's a better generalist than the twin-linked lascannon or assault cannon turrets, but it's not suited to a specialist role.
My final verdict is that the best turret is the one suited to the role you have it playing (surprise!). If your Razorback squads are going to be played aggressively, your best option is to take twin-linked assault cannons, because you can move 6" and shoot, giving it a 30" threat range which is plenty for an army that is advancing. If your Razorback squads are going to be played defensively, like mine, you're best off with the twin-linked lascannon variant. I use my Grey Hunters in Razorback to support my Long Fangs if my opponent has outflanking/deep striking threats, so the Razorback is perfectly happy moving 6" or less a turn and pumping out lascannon shots. If troops in Razorbacks occupy the majority of your list (Razorback spam ahoy) then you're probably best off taking the lascannon/plasma turret. Because you have more Razorbacks you can afford to have slightly less optimal shots in any given instance, and because your game may be more defensive or more aggressive depending upon your opponent, you have the versatility to be moderately effective in either instance.
