great advice.
bass is going to be the more felxible in most cases, not because the instrument is easier or that more complicated things can't be done with it, but i suspect you'll find yourself limited more by the drummer and guitarist/keys in determining your collective style, they will have a much harder time skipping around styles comeptently, assuming them close to your calibre of experience.
as someone without a lot of stylistic expertise, i find it easier to let my intuition take over and rely on scales and modes to keep me in check. the better i understand modes and scales that are being played, the style of how to play them is usually the easy part. but i'm also a drummer so i guess rythm, and rythm improv, comes a little easier than some might find, while i have no serious chops.
because of my own impediments, i always find picking notes to be the hardest part of improvisational (jam) stuff. but the closer to basic blues i feel in command, the easier the rest of those concerns all become.
one thing i do constantly is simply go up and down a blues scale from the B string up two octaves. i go up and down her at least 20% of the time the axe is in my hands and the more i do, the more natural and intuitive improv becomes later on that scale, or other blues scales - even getting to the point where i can jsut intuitively play what i hear in my mind's ear.
even more exciting, when i start to look at different modes, they suddenly make a lot more sense and feel easier to work with, even from memory.
the other thing i do is
jam with myself, as much as possible. i have a digital workstation, very expensive, so this is very easy to do - but if there's anyway you can recreate this or get your hands on one, i can't recommend them enough. beyond being addictive, they are probably the most amazing practice and motivational tool i've ever experienced.
i start by laying down a song form a CD i like or a drum track from a drum machine, or i just drum for a few mintues to a metronome. then i go back and lay down a few bass tracks to suggest a foundation, then i go back and lay over high bass stuff or try singing to suggest melodies (i can get a guitar sound with my voice putting the mic through bass effects processor).
deep.
sorry, too much coffee, just love to see people talking about improv.