Mmmmm...that's a great question. I've been using graphics software since '88 and have done layout and seps out of every major program out there.
I tend to use photshop, painter and freehand to do all my work after the intial illustration is done.
Text layout and effects (twisting bending etc) is more easlity done in a vector based program like freehand or illustrator. I then take that text into photoshop working full size at 300 dpi.
You can assemble projects in Quark, but I would use photoshop or freehand/illustrator to do the main bulk of the designwork.
I have heard many people say that they get a little sawtoothing (fuzzy edges) when they convert thier seps to bitmaps, but I don't really have that problem. What sawtoothing that does appear (and it' very minimal) usually is not noticeable when it is finally screenprinted. On 4C process offset jobs I do everything out of photoshop and painter.
The only reason I recommend photoshop, is it is my personal preference. You can do what complete posters in any of those programs. I just find that my tricks for trapping out the colors are more easily done in photoshop. I then take the seps into freehand for imaging, additional small type and tweaking.
Good luck in first poster and if I can help in any way feel free to write. I love to see people get involved in doing posters. It's an addiction...
Jeff Wood
Drowning Creek Studio
jeff@drowningcreek.com