Yes you should go and get the RGB monitor.
Like the others have said, the A520 never did a particularly good job of video output of either composite or RF. Since I am guessing you are in the USA, you won't be able to display PAL video on your TV (at least not in colour).
Where as with the Amiga RGB monitor (is it a 1080 or 1084 that you are considering?) you will get sharp, crystal clear video and the ability to display both NTSC and PAL video (good for euro games and demos).
Unless you absolutely need a bigger screen than the Amiga RGB monitors, you would be silly to pass it up.