As John suggested, thicker will be stiffer, but for the purposes of a bulkhead you don't need it to be stiff. Its probably actually better if the bulkhead has the ability to flex.

I agree completely, in general.
If you are using the forward bulkhead as a footrest, it's better if it's stiff. Repeated flexing of a thin bulkhead (with foot drive) will really put your filleting and bonding job to the test. Other than that, the bulkhead just has to withstand the pressure of water in the cockpit or gear stuffed into the compartments, and a few layers of glass laid up into a panel will do the job. An advantage of an uncored bulkhead behind the seat is that it can be curved to reduce the size of the 'corners' where water can collect....though those corners can be handy spots for stashing gear.

Even a very thin, flexible-feeling glass bulkhead will be very strong/rigid in the vertical direction if it's bonded all round to the hull and deck. Try to crush the end of a tin can to get the idea, or imagine that the bulkhead is the web of an I-beam.

Most wood-core boats are so stiff that the bulkhead doesn't add much of a 'hard spot' when the boat slides over a rock. With a flexy plastic or even thin glass hull, you'll usually see foam bulkheads which are much thicker and more resilient than a hard bulkhead.
I have seen a classic fracture right across a thin Kevlar hull at the aft bulkhead, so 'hard spots' are not just theory.
I've also repaired a few glass boats where the flange on the glass bulkhead popped free from the hull, allowing water to leak through. That gap is also a dandy spot for collecting sand and mold.