I extended our usual week-long stay to a 15-day stay a few years back [a year after Potter opened]. We spent roughly half our time at Universal.
Although we both enjoyed the experience, neither of us have been in any great hurry to go back. To us, Disney is an experience while Uni is an amusement park. We prefer Disney.
You're right about the expense. Buying tickets to both is a lot more expensive than just extending your stay at one or the other. If cost is a factor, doing one or the other is probably a wise choice.
I'll never recommend to someone that they avoid Universal, it's great for some folks, just not my son and I. There are many for whom Universal is the better of the two and I wouldn't presume to tell them they're wrong, they just have different interests than we do. We'll probably go back a year or two after phase two of Potter opens [I've got no interest in dealing with the post-opening crowds] and we'll catch up with the other recent additions. But WDW will always be our first choice, Universal will probably be a once-every-decade or so add-on.