I notice that no one has yet replied to you, so I taught I would write a bit based on my knowledge and hope to get other people more knowledgeable or that happen to have experience with both guitars to weight in.
I own a Premium RG1XXV shocking Pink guitar, and I am personally very happy with it. I have never used an RG550 from the early 90's, so I can't really judge on how they would exactly compare in sound quality and fretwork.
What I could mention is that the MIJ 550 based from a quick search online, comes with the Edge tremolo and is well known that these guitars from that period of time have a really good quality of craftsmanship.
They also have the features that were popular at the time, in contrast to the Premium that has seen the evolution of the RG with updated revisions to the classic design by incorporating the ibanez AANJ neck for extra comfort, adding the neck volute and multiple woods to strengthen the neck from cracks which the older design is prone to suffer. Also the RG1XXV comes with the Edge-Zero II with the ZPS3Fe system on the back to allow to adjust the springs of the tremolo with a knob.
At the end of the day the main actual difference comes down to the Classic RG550 design and features vs a more modern version of it, that have the crazy neon colors and happens to be made in Indonesia instead of MIJ. I must note that there has been plenty of talk online and here on this forum about the disparity of quality with the Premium guitars. That said is hard to know what exactly you are getting until you try the guitar. I bought mine online, so had no way to try it before buying it, but after doing a small tweak on the set up upon arrival, the guitar played great and has been ever since. I am personally very happy with my RG1XXV. (Don't know how the fretwork compare, but hopefully someone con shed light on that). I am inclined to think that the MIJ is superior, but lets hear from others who know more about this topic.
Both guitars are good on their own way and at such a price. It boils down to what you value and like more about either one and what they represent to you.
For me for example the deciding factor when I had to choose between this guitar and another was the guitar having the look of matching color for the body & headstock as well as the shark tooth inlays of the white RG450 that I was drooling as a kid since I saw that guitar on the Ibanez Catalog of 1999 + the Neon Pink like color of the early JEMs that I also remember from old guitar magazines. So this guitar had the nostalgia factor of 2 aspects I liked of 2 different guitars. The other guitar was the Sea foam green Jem. But at the end I went with the RG and do not regret it. (Price was also a factor in my case... as either I buy now and enjoy now or wait for months until I save enough for the other more expensive guitar)
I hope this information helps you decide on what matters most to you.