We did this just 6 weeks ago...the decision was helped a lot by one night of no availability at OKW during our week-long stay! We booked that night at the Royal Pacific--it was smack in the middle of our week (Tuesday night)...
We had two villas at OKW...one we could and did keep for the entire week...the other had the "missing night..."
We left OKW, via a cab we called that picked us up outside our Turtle Pond villa, at 6:15 a.m. My friends/guests left their luggage in my villa and off we went...(when they got back Wed evening, they were given the exact same room! We got a kick out of that--)
All four of us (two adults, two kids) were booked into the Royal Pacific room, so when we got there we all got the early entrance/front-of-the-line passes...I wound up returning to OKW to my villa Tuesday night (too much togetherness is too much togetherness!), while my friend & her two kids (teenagers) stayed at the Royal Pacific...
We LOVED Harry Potter land...would go back again in a heartbeat, and spend the same HUGE amount of money to do so. Less impressed w/both Islands of Adventure itself and Universal Studios...liked them, but wouldn't return any time soon were it not for Harry Potter.
All told the costs were high: the 2-day/2-park tix were around $90 each; the cab from OKW to RP was $45 (return was higher--$55, because the hotel put me in a town car rather than calling me a cab...learned to insist on a cab "the hard way!")...the hotel room was just shy of $300...so, for the four of us, total cost was just shy of $1000...my personal cost was about $200 (park ticket and extra cab from/to RP-OKW for Tuesday night)...my friend paid for the cab to/from OKW-RP Tues morning and Wed evening, and for the hotel room (I'd provided the OKW accommodations for the whole week, so I didn't mind accepting that--and she also paid for 3 2-day/2-park tix for herself and her kids)...I'm not counting the cost of food cuz we'd have incurred that cost at WDW, too...we didn't find food prices or choices all that different (altho I like WDW food better, that may be because I'm biased, or because I know it better and therefore know where to go to get "the best" food)...
RP is less expensive than the other US/IoA hotel choices, and a bit further away to walk over to the theme parks (like maybe 10 minutes instead of 7 or so...not a lot of difference, and a pretty walk thru landscaping and water/canal path)...Mears transportation is considerably less expensive than cabs for a single person--less of a difference for a party of four--and, cabs really cut down on the time it takes to do the move...RP is a nice hotel, but way less wonderful (we thought) than OKW (or any of the WDW
DVC resorts)...
I'm REALLY glad we got to Harry Potter (we went both mornings, and the early entrance made a HUGE difference...we were there at the end of June, and in the midst of a very rainy week, so crowds were probably about as high, season-wise, as you'd expect, but lessened due to the crummy weather--we were wet almost all the time, all week, and lived in our ponchos)...and while I certainly did not dislike US or IoA, I'd not give up a minute of Disney time to go there, were it not for Harry Potter...
When doing the planning, and before finding out about the missing night of availability at OKW for one of the two studios we needed, we thought about going to US/IoA for the start or end of our trip...but that messed up Magical Express (which made the cab cost to/from WDW a wash)...
Anyway, that was our experience--expensive but VERY worth it, to us!
Good luck!