I'm with the "play pro bass on expert (no-fail) first" crowd. Full chords on pro guitar are really intimidating at first, and for pro bass you don't have to worry about this (the worst you'll get are maybe power chords and octaves), so you can focus on developing left hand awareness and getting comfortable with moving around the neck.
The other thing I must stress is that you will NOT be able to just jump into a song and start playing the way you may be able to with legacy guitar or drums. There's just too much information coming at you to be able to do so. I've been playing pro since RB3 launched, and sightreading pro guitar is still really hard. Instead, I recommend that when you want to play a new song, first (1) go through the trainer for that song, and optionally (2) play each section in practice. Most songs have repeating patterns, and the trainer will familiarize you with these, taking a huge burden off the sightreading when you actually play the song. Playing through in practice (maybe slowed down to 70%) helps to learn the structure of the song, and to repeat shorter sections multiple times in order to lock them into your short-term memory.
If you really hit a wall, then by all means, lower the difficulty. Some songs may just be much more difficult for you than similarly tiered songs because they require certain techniques that you haven't really developed yet. The more you play, the better you'll get. As you become more familiar with the instrument, you'll find that what seemed impossible before suddenly starts to make sense, and is playable. Little by little, you'll see improvement in your playing. Just be patient and persistent, and you'll get there.
One last thing: you might try playing both with a pick and with your fingers when doing pro bass. Some songs, especially some of the hardest pro bass songs like Llama and Antibodies, I believe are only really playable with your fingers, whereas many or most of the pro guitar songs really benefit from playing with a pick. For pro bass, I find that a default position of thumb on the lower two strings (E and A), index on the D string, and middle finger on the G string works pretty well.