Yes, you can use the pool at all three hotels if you stay onsite. We have been to both hotels and really prefer the Hard Rock. That is just our personal opinion though, there is nothing wrong with the Royal Pacific. The rooms are a bit smaller at the Royal Pacific and the room decor a bit dark. But the common areas of the hotel are absolutely gorgeous.
The Hard Rock was our first experience with Universal, so it really feels like "home" to us. I still get excited like a little kid when we pull up to the Hard Rock, I love the music and the whole atmosphere. The kids love the Hard Rock and don't want to try anything else. They certainly would if we wanted them to, but after DH and I stayed at the RPR by ourselves a couple of times, we both agreed the Hard Rock is better for them. The pool is always busy with things going on and is just a really cool place to be.
As for conveinence to the parks, Hard Rock is closer, especially when you can use the back exit (I'm not sure if you still can). It is really just a five minute walk. The Royal Pacific is still very conveinent - it is about a ten minute walk, the walkway is very pretty. There are times though when I usually say "enough already! are we there yet?" LOL But, really it is not far.
It really is just a personal preference, I think you will enjoy both places. Is it "worth it?" as you say to switch to the HRH? Well, the extra $30 per night certainly is to us, but may not be to you. Take it Easy!