Assuming you're on a jailbreakable version of iOS, my advice would be to jailbreak your 8 Plus. The 6 Plus is probably the worst phone to jailbreak due to the limited amount of RAM for that size device. You'll most likely end up getting multiple restrings/crashes into safe mode which becomes very annoying very quickly.
It is very difficult to actually brick your device by jailbreaking. If you don't like the jailbreak or something goes wrong, you can always plug your device into iTunes and update to the latest iOS to remove the jailbreak. Just read up before you proceed so you know what you're doing before you do it.