I like RS2 (though only got about 10 hours in) and I can understand anyone who thinks yet more highly of it. RS3, by contrast, I found to be one of the most banal and disaffecting games I've played in the last six years, being both unpolished and directionless, when usually games compensate for one of these qualities with the other.
The only way to compare playing RS3 to climbing a tall mountain is if said mountain was situated in the Netherlands and you were going by cable-car. I felt like I had to think about strategy (beyond grinding for 5 minutes to pick up an obvious counter-skill from random encounters in the boss area, i.e. Gaze/Net/Hypnosis) exactly once, vs. the first Sinistral, in all the 40 hours of trudging through that game. To be fair, I lucked out by making Rukh learn wind magic early. Still, there were countless other means to win almost every random battle before the enemy could even act; neither tactics nor resource management were remotely a concern through the vast majority of time I spent.
RS3 also ultimately fails at telling (or making you tell) a compelling story -- even obliquely via its world -- despite that its backstory and the web of relations between the initial party members provide an intriguing setup. RS2 doesn't seem to start out all that special, but its overarching motivation (and the both mechanically and aesthetically brilliant theme of time-skipping, which fully justifies the title of "saga") gradually made me invested.
In general, I picture that RS2 is kin with 7th Saga, another game that isn't so much truly difficult as beautifully stubborn and -- because I cannot say it better -- otherworldly right because it sticks to "conventions" that it presents with a strange almost-understanding. I didn't find that in the sprawling, cosmopolitan and quite cartoonish RS3. Of course, 7th Saga isn't nearly as much of an "open world" RPG as RS2/3.
tldr: anything RS3 does, RS2 does better. RS2 isn't a great game, but an interesting one that deserves its reputation of unconventionality. RS3 only seems to be unconventional, but in effect, it plays the same as RPG shovelware anyway and, beyond mechanics, I was unmoved by most of its artistry.
Btw, both games pay smart homage to FF2 ambush closets, which I think is mighty impressive.