i play both and let me give you a quick rundown on which one you should play.
Rs3, very easy to max, bossing takes a lot more skill on rs3 then 07, rs3 is mainly pvm orientated rather then pking. Community's pretty nice when it comes to newer players if your not begging for items. Ive found plenty of nice willing people to help me when i returned a while back.
07, takes some time to max, but tutorials will pretty much carry you to the fastest routes, more pking orientated, pvm is fine, but not a huge learning curve. More toxic community ngl, lots of scammers. It feels way more rewarding to build from the ground up, stuff is pretty cheap on rs3.
So tl;dr, 07 for a long term grindy game with raids/pking endgame, rs3 with a little more on the causal side, more pvm orientated then pking.
Personally, i prefer rs3 over 07, both is fine but i just prefer the late game pvming on that game. its easy to get into a community if your new, 07 looks for elitists for most high end clans.