Coming all the way from BC, we always do both WDW and Universal. We have also done Seaworld for a day on some trips. We usually take a cab from WDW to Universal but next time we may stay onsite at universal for a couple of nights.
I'm just researching this too. We did WDW for the first time in Nov 2015 and loved it (we have many DL trips under our belts) but DH mentioned wanting to try Universal too next time. DD is now 7 and will turn 8 during our next trip if we go when planned. I figured a 4-6 at disney, and 2-3 at Universal.