I am going to make the unpopular suggestion that I will readily admit is mostly due to paranoia. I personally always have to have a ground cover, regardless of the material of my tent or bivy or whatever. So knowing that I am more comfortable always using a ground cover, be it a silnylon or Cuben tent/bivy, I took a step back and looked at the advantages of each fabric a bit differently.
The whole abrasion thing with Cuben is not really an issue if you have a ground cover. I use a double ground cover most of the time, which is a DIY job of a garbage bag and a cut up piece of space blanket with duct tape to hold them together. Plus a small Cuben stuff sack, this double ground cover weighs 88g. This gives me the peace of mind knowing that my Cuben bottomed bivy (which is what I use most of the time) has two layers of protection. Of course I am like most of us here, in that I am also careful and make sure to clear the ground and all that. I even lay my ground cover and then lay on top of it and feel around for any pieces of debris I may have missed, and this technique really ensures I do a good job clearing ground.
There is another item of gear that comes into play here, which is sleeping pads with any kind of air in them. If you use them, they also benefit from having a little extra protection with a ground cover.
But seeing as silnylon is slightly heavier, and also slippery, and I read in a few threads on here that sometimes with pressure it can seep water through, and given my personal issue of always using a ground sheet, Cuben wins out for me.
If you don't care about being extra cautious with your floor, go for siynylon and no ground cover. If you want/need to be extra cautious and will usually or always use a ground cover, go for Cuben. If you are even more paranoid/cautious than I am, go for silnylon plus a ground cover and just deal with the extra weight.
It's worth noting that ground covers are not the heaviest addition either. You don't have to be crazy like me and use a two layer one. In my 4 season kit for winter/frigid/very cold temps I use just a garbage bag as a ground cover, as I don't use a tent or bivy in these conditions, and it only weighs 42g plus a tiny Cuben stuff sack. Even a really tough Tyvek ground cover is going to be around 3-4oz I think. I think once my double ground cover gets worn out and needs replaced, I might get a Tyvek one in the future.
Good luck whatever you pick.
EDIT - fixed some typos, added a few details