That's funny, I asked my husband the very same question when we went to Orlando for the 2nd time. You see, we had gone a couple of years earlier on what we assumed would be a "once in a lifetime" trip and wanted to see a little bit of everything Orlando had to offer. We went to SeaWorld twice (2nd visit was free if you went back within 7 days of the first visit), one full day at MK, plus a second visit to MK for
MVMCP, and 5 days at US/IOA because those tickets were so much less expensive than the comparable Disney tickets.
The 2nd trip we were going with friends, and it really was their "once in a lifetime" trip. We wanted to see all 4 WDW parks this time and they wanted US/IOA as well. That was the year they introduced the 10 day MYW tickets and I asked my husband "How can I justify paying nearly $400 in additional costs, when it would only cost $13 to add the extra days to our Disney tickets?" His answer was "very easy, because if we're going to Orlando, I'm going to Universal!"
I was thrilled because the truth of the matter is that we love, love, love Universal and Islands of Adventure. Our happiest day of vacation is always at Universal, and our worst always seems to be a DHS. The area that is now WWOHP used to be Island of the Lost Continents and I thought there was more "magic" in that area than in all of WDW.
When we go in February, we are only doing one WDW day, and that is only because we happen to have one day left on the 10 day MYW PH Plus tickets we bought in 2006. It will be a full day of hopping from park to park to try to do our favourites...something that is easily done at Universal Parks because they are a 10 min walk apart, and there is only two of them. However, they manage to have within those two parks a ride or attraction that matches the WDW equivalent. US/IoA's version, however, is bigger, faster, edgier and more thrilling. For our family, US/IoA wins, hands down.
Now, Tower of Tower, Soarin', and the Safari do not have similar attractions, and I do have a preference for Mickey's Philharmagic over Shrek, so we still do some WDW stuff when we make the trip to Orlando.