The average user, even a power user, won't notice the difference between PCI-e and SATA SSD's. I'd save your money and buy whatever's the best value you can find for the size you need. Sandisk X400, Crucial MX300, whatever... you can buy 'em in M.2 form factor so not sure why you're worried about cables?
(M.2 form factor supports both PCI-e and SATA, just need to see what your motherboard supports- very likely it's both PCI-e and SATA)
If you don't care about write performance then why worry about it? Also a good TLC NAND equipped SSD under even most power users' workload is not likely to exhaust its pseudo-SLC cache, and hence you will never notice the performance drop because you won't ever get that far. If you're reading benchmarks at Anandtech or Storagereview or whatnot where they do show the performance hit, well, it's true, but how often are you ever going to push that many sustained I/Os?