If you like HP, you will love the detail at Universal...it is really pretty amazing.
We love Universal even without HP. That being said I won't go unless we are staying onsite as that is a big part of the appeal for us. The resorts are deluxe and they are all right there next to the parks (walking or boat ride distance). The biggest plus is the onsite unlimited guest express pass as it can't be beat, you will be completely spoiled. Although it doesn't include a few of the rides, you do get early entry for Harry Potter (note that the newest HP rides may not have early entry for your dates - we don't know yet how this will play out in the long run).
I would suggest 2 days (one for each park) so you don't feel rushed.
We just feel super laid back and enjoy it thoroughly over 2 days with express pass.
We've done Universal as the second half of our trip in the past, but our upcoming trip (in Oct. as well) we are doing Universal first so I'm curious to see how that works for us for a split stay.
A few tips if you go and/or are staying onsite:
If you just do one day and don't stay onsite - look at the universal crowd calendars and pick a weekday so you can get more done. And arrive early.
HHN will be going on in Oct. and crowds could increase on the weekends or on other HHN nights (I think Thursdays are the other night) and park hours are different for the HHN nights. The HHN calendar is available on Universals website and so are the park hours. HHN is a separate hard ticket price similar to
MNSSHP, you can do a stay and scream ticket for a pretty fair price.
It is always cheaper to book your room directly with Loews and buying tickets separately rather than doing any of the "package" deals.
You can stay 1 night onsite but actually get 2 full days of express pass if you check in early enough to enjoy the 1st day. You can check in very early, leave your bags, get your "room key" and go to the parks and use express pass. We've done this a couple of times we've stayed.
Look into the price of an annual Power Pass for at least one person in your party (noting blackout dates) to see if that makes sense for you as a lot of the time the hotels have great AP discount rates and the Power Pass qualifies. These rates come out about 2-4 months in advance for future dates (Oct. has some showing right now). It is always a good idea to check this out if you are staying for 2 or more nights as it can save you money. You can even book as APH without buying the pass first by calling Loews or going to their website and using the APH code - you can buy the pass later if the math makes sense so you have it during check in. If you have an annual pass you can get a discount when purchasing additional tickets - so that is something else to check out as well before buying all your tickets.
Royal Pacific is very nice and it is a very good value for a deluxe category hotel (comparable to Disney deluxe). Portifino is our favorite and is the most expensive... but for a very short trip I would probably choose Royal Pacific. Hard Rock is closer to the parks but it doesn't appeal to us and we haven't stayed there, so I can't comment on it.
The only downside to onsite are the parking/resort fees added daily to the rate, I think it is about 22.00 now but I figure I would have to pay for parking anyway if I didn't stay onsite so the cost is a write off for me.
There are some great restaurant choices at the resorts and City Walk. You can reserve a table for Mythos as well as some other restaurants via Open Table. We enjoy the food choices at Uni better than WDW overall.
Try a Butterbeer, they are surprisingly good!!!
Other things that we never miss: The Simpsons, Transformers, The Mummy, Spiderman - all very cool rides experiences but not really "thrill" rides.
Enjoy your WDW and/or Uni trip!!
