ultimately it depends what your preference is. if it were me, i would go via DFW only because i prefer the longer flight time to enjoy and rest, though i would not be ok with a 1h20m connection in LHR on separate tickets as MCT is 90min when switching terminals (seems you are on AA metal DFW-LHR?)...so that would mean a longer connection in LHR for the later GVA flight but you can use the T5 CCR for some food or showers or whatever to pass any additional time.