jinx: it really isn't that simple. you're skewing high for GF. remember that GF/DM contain plenty of low-level songs that advanced players never bother playing any more. given that GF/DM have 'extreme' songs rating from ~20 through to 99, and basic songs rating as high as the 80s (or possibly 90s, I forget with the new songs), and the range in GH/RB is almost as broad (although there's no really hard songs at the 'easy' or 'medium' difficulties), comparing difficulty levels is fairly meaningless. Saying that GF BAS is more or less like RB Hard is fairly absurd - a level 10-20 GF song, of which there are plenty available at basic level, is much easier than almost all GF/RB songs on Hard.
It's better to just compare the distribution of absolute difficulties available; GH/RB games tend to have far fewer really difficult songs available, i.e. their distribution skews towards the easier end of the scale, with only a handful of really challenging songs in each game. That's all you can really say. But there are definitely a few songs in the GH/RB series that are extremely tough (both drum and guitar). | 
