A couple of points to help you:

1. You mentioned that you can purchase the Express Passes and that they would be the same as the EPs you'd get from the Premier hotels. It depends on which type of EP you are purchasing. The Premier hotels come with an Unlimited use, multi-park EP. That means you can use the EP as many times as you want on any ride in both IOA and US (note that Volcano Bay EPs are a completely separate EP and does not come with the Hotel stay). When purchasing an EP, there are various options: Single or Multiple Park and Single use EP (less expensive), Unlimited EP (more expensive.) I'd say than in most situations, the cost of purchasing 2 Unlimited Multi-park EPs would start at around $240/day for 2 people (with the price going up for more crowded days). So 2 days of Unlimited 2 Park EPs would be almost $500 (or more). So add that cost to your off-site hotel stay.
2. The Water Taxis and buses are all in full swing. And Universal is much, much, much better with transportation to/from their on-site hotels than Disney. In most situations, you won't wait more than a few minutes for a Water Taxi and the ride itself is fun. A lot of the Taxi captains have been known to entertain during the ride. And of the three Premier hotels, PBR is the furthest away from the parks, but even so, a normal walk would be around 15 minutes (or down to 10-11 minutes if you walk through the HRH parking lot for a short-cut, but of course your walking through a parking lot rather than the nice winding walking path).
3. If you stay off-site, getting to/from your hotel will take more time. If your off-site hotel provides shuttle service to/from CityWalk, they will very likely have a very rigid schedule. For example, I've stayed at the JW Marriott before and they do have a complementary shuttle, but it only runs 3 times each way during the day. It won't be easy or quick to head back to the room and then back out to the parks again. Staying On-site makes it very easy to head back to the room for some pool time after lunch and then maybe head back out to the parks for a later afternoon park run.
4. Staying on-site keeps you in the Universal "bubble" for most all your trip. For us, part of the experience is not only the parks, but CityWalk and all the restaurants there.

I've stayed at the FFI and the Doubletree across the street from Universal's entrance (both of which are actually within walking distance especially now that the walking bridge over Kirkman road is open). I've also stayed at the JW Marriott and even some of the Marriott MVCI properties before (for the extra room they provide), but we have decided that even though I can't use points at the On-site hotels, they are worth paying the cash for. I've also tried staying at some of the on-site non-premier hotels like Cabana Bay. They are nice as well, but we tend to stay at one of the 3 Premier hotels the vast majority of the time as the Unlimited EPs and the convenience just can't be beat.

And of the 3 on-site hotels, we like PBR the best with its 3 different pools and on-site SPA. Plus it has the best Club lounge as well (and I quite often splurge for that as well.)