I'm the same as RDI ... I too run both 2.5 & 5 at the same time. Not because I want to, but because my house is a ranch, built with concrete walls in some places that divide sections of the house and although I have 3 WiFi extenders, and the 5.0 is significantly faster than 2.5, the 5.0 range is significantly less than the 2.5 and my phone is set up to jump from the 5.0 to the 2.5 whenever I'm in the farthest reaches of the house.
I agree that if you can use Cat 5 or 6 from the connection on the pole or utility box outside to an upgraded modem, (and WiFi Extenders), you'll have the best connection options available.
As for the phone itself ... I don't have ANY issues with the WiFi connection once IPv6 was disabled on the Modem. And don't take the Cable Companies word that they don't have that activated, we found that 2 of the 3 modems it was activated when we turned them on even though the Cable Guy and company said they were disabled.
No problems with WiFi reception at all with this formula.
Good luck.