I guess it depends on how you use your phone. If you find yourself browsing the Internet with multiple tabs at a time, switching between them often, or switching between multiple apps at a time, wait for the 6S. App performance itself isn't necessarily affected by RAM, it's how *multiple* apps can stay running without refreshing. When apps refresh, it can sometimes be extremely frustrating because it can cause you to lose progress, or lose your place in a webpage... Quite irritating depending on the situation.
I would say wait for the 6S even if RAM isn't an issue because the camera is said to be MUCH-improved, the phone will be much speedier, and force touch may be implemented! Always buy things at the earliest in their life cycle, saves you money down the road when you don't have to upgrade AGAIN because your phone is already slowing down. If you absolutely have to have a phone now though, you should still be okay with a 6 or 6 Plus, as long as RAM isn't an issue as described above.