Early in the morning and closer to closing is when I've found it pretty easy to get on most of the rides. When my friends and I go, we hit Islands of Adventure first as close to park opening as possible. Depending on what you're looking to do, it should be fairly easy to hop on most rides with minimal wait. With that said, we're not Harry Potter people, so we don't really do much in that part of the park, but it is the area that fills up the quickest, regardless of the time of year. The early times let you hop on the rides in the Marvel section of the park (Hulk, Dr Doom's Fear Fall, Spider-Man) with minimal waits. After about 11am, the wait times start to jump up. At about 4-5pm, the parks start to thin out as the local crowd starts to head home and the wait times drop again. After five, things really thin out and there are barely any wait times.
If you get their equivalent of a park hopper pass, you can bounce between the parks on the Hogwarts express. Besides being a neat "ride", it's a nice way to get between parks while sitting down in an air conditioned cabin. In January, it probably won't be that hot out, but sitting down for 15 minutes after a lot of walking and still getting somewhere is a nice treat in itself. The main Universal park seems to have more motion simulation rides than standard coasters. If those are your thing, it'd probably be a decent idea to start at US and then work your way over to IoA as you hit all of the rides you're looking to do.
My friends and I did a two day Universal trip and managed to see a fair amount of the park in that time. There are pretty decent sized areas of the park that we were able to skip out on because of their focus towards the younger crowds, the the whole Seuss area, and the older comic type area between the Marvel section and the Jurassic Park area. Also, we're not Harry Potter people, so our time in that section was limited to the Dragon Challenge roller coaster and hopping on the Hogwarts Express to get over to Universal. If you're skipping the shows at Universal, it's also a more manageable park. With a little planning (not anything like WDW planning), you can see a good deal of both parks in two days.
I have an AP for Universal and have gone a few times (it helps living 2 hours away) and have never purchased the Express Pass. I think the longest wait we had on our last trip in April was just over an hour for the Rip Rocket Rollercoaster because we would make it to that area of the park right around lunch, when it's packed with people. The early park time had us on Hulk in about ten minutes and then hopping on the ride again when we were done. Also, if you don't mind who you're sitting next to on the roller coasters, use the single rider lines whenever possible. That'll cut a decent amount of time off of your wait, and can be as quick as express pass lines in some cases.