Cuben "cloth" really isn't a cloth. Silnylon is. Many of the percieved differences between the two can be traced back to the simple construction methodes between them. The raw strength between silnylon and CF is mostly percieved by the different weights of the material (and coatings) between them.
Silnylon is a treated cloth. The treatment is silicon rubber. The cloth is made of threads. The threads are made of nylon. It is woven into a cloth, often with other materials used as "ripstop" fibers or a simple heavier thread. The treatment is then spread lightly over the cloth, often by rollers and/or knives, where it penetrates the the threads and gaps caused by weaving leaving a thin film to waterproof it, incidently making it hydrophobic and slippery while reducing the texture of the woven cloth. Properties we exploit in rain for excellent waterproofing. A more conceptual view of silnylon would be a silicone film, reinforced with nylon cloth.
Cuben is made by taking two(or more) layers of waterproof plastic and sandwitching them over a reinforcing fiber, often spectra, kevlar and carbon. These are pressed togther under very high pressures until they actually bond. It is stiffer than a woven cloth and doesn't have the streatch. Conceptually you can view this as a plastic film reinforced with threads.
The difference between the two, because of the actual construction methods, becomes very large.
Attacment of multiple pieces or loops.
A thread through cuben will elongate a hole till it anchores on a fiber because fibers are sparsely interspersed. This also means one or two can tear before a couple others can bind up together supplying the stopping power. So, holes get larger when sewn, especially under heavy loads. A thread sewn through silnylon has a high density fabric that "catches" slippage. It will stretch, elongating a hole, but much less than cuben.
To avoid this problem adhesives are used. These have different rates of stretch than the plastic used in cuben. Under load, it will crack, potentially leaking. It can create a stiff seam that can crack, too. So, a non drying adhesive, like a tape adhesive, is used. But this can creep under loads. So, simply stitching it and bonding it produces a joint with the best characteristics of both, simultaneously reinforcing the worst characteristics of both types of joins.
On silnylon, reinforcement is not usually needed. But this brings up an interesting side point. Seam sealing, usually done with a thinned silicone adhesive on silnylon, and often done new, is not as effective as *after* holes have already stretched open and fibers have moved, slightly.
Repeated loading, such as with a tarp, will give different results, also. Cuben is, for all intents and purposes, a plastic film. It can stretch permanently under load. Eventually it will fail, but more importantly, it will fail if scratched. A good example is thin plastic tarp. It is quite strong untill a scratch, nick or other defect is made in it. Then it tears quite easily. It is almost impossible to avoid all abrasion while camping. So, unless you use heavier weight cubens (multiple layers,) small holes/leaks, or "slices" may appear. These are not easy to repair with cuben. Patching rather than sewing is prefered, duct tape is good for that. Wind hammer can cause increased streatching. A not quite perfect pitch and leave flapping, causing "impact pressures" that exceede the material's normal strength.
Silnylon, while not subject to surface abrasion, *is* subject to stretching. Fibers in the cloth can become displaced opening previosly sealed seams, pole points (often reinforced,) loop tieouts, and, the fabric itself. This can continue to extremes. Example: My tarp, a 9'x11' silnylon has about 3" of stretched material near the center from constantly loading it over a hiking stick. Loosened coating is broken, tiny, inconsequential leaks now leak through, etc. Inside, it is easy to see where it is stretched and where it is not with a good light behind the tarp. This can be repaired (and has been twice) with a layer of silicone/mineral spirits over it. Normally, impact pressures are more distributed, causing little extra stretch.
So, while both are strong, general durability for silnylon is greater, if only because of the *way* the two are made. If you are seeking UL weights, you expect decreased durability, soo, this isn't usually a problem. This is a philosophical difference, subject to interpretation. I do not recommend one over the other.
I hope this helps!