I like both of these fish in completely different setups. Some fish don't mix, others do. My 4 DP's are doing fine with about 9 Flagfish in a 55 gallon, but this tank used to be a more diverse community and I had Neons, Ottos, Corys, Flagfish, and a Betta. Both setups worked great, but I would definately not add a Betta to my current setup!
You really have to decide what direction you want to go with the tank. Just DP's? DP's and other fish (sometimes difficult)? Just a Betta? Betta and other fish?......which do you choose?