Hey I have cycled from Tengchong to Dali, opposite of what you plan to do. I suggest a few more days, there are some large mountains in between the two! Tengchong is surrounded by geographical barriers from both sides. If you decide to go to the Nujiang (I have cycled that as well) I suggest taking the bus up and cycling down. It takes two full days to cycle all the way down to Liuku from Gongshan, not something you need to do twice! I have not been to Ruili, but it sounds like the more suitable choice, as you would be backtracking from Tengchong if you went up the Nujiang. Add more days to your trip, if you havn't left yet that is! Is this your first time touring?