Netgear C3700 and R6250 (AC1600) - Best config or should I upgrade? 100 MBPS Comcast

I currently have the C3700 set up as modem/router wired to R6250 (AP). I recently increased my Comcast speed from 50 Mbps to 100 Mbps. I wanted to know if it would be better to upgrade/replace the C3700, R6250 or both and what recommendations. I tried to upgrade to the CM600, but I couldn't get the R6250 to work and had to use my N750. I gave up and just went back to old config. then the R6250 was happy as crap and greened up almost instantly. Any suggestions would be great. I have a 2 story house that the R6250 beams very well...including outside a bit like the driveway and back yard. Would prefer not to spend $300 on any one item though unless I need too. One last note...I do honestly think that it had something to do with how Comcast set up my new CM600 though, but not sure since the N750 worked.