The basic difference between the PBX & turrets is the PBX is designed to have many users share a limited number of outside lines, while turrets are the opposite - individual users requiring large numbers of outside lines. The dealerboards give traders quick one-touch access to dozens or hundreds of lines on their desktop, and they're capable of providing speakers, multiple handsets, and other 'gotta close this right NOW and don't get in my way!!' features.
If you have a smaller, relatively sedate trading team, you can probably get by using PBX or key system features - one button per line (square key layout), and use speed dials if traders can wait for them to dial out. If it's a fast paced team, the turrets will keep them happier. At a cost.
You can do PBX sets for about $1K / station, but turrets can easily go for $15K per seat. But the cost is meaningless compared to one good trade, right?
Good luck!