My 1/2 Mb broadband was great, then they upgraded me to 2Mb for free, and it drops out quite a lot and I have to redial. I'm thinking of asking for a free downgrade to 1Mb.
With a leased line you get an actual length of copper wire they come and connect up to the exchange that carries only the computer signal as opposed to broadband that shares your voice and computer on one line. The "modem" for a leased line will be more expensive as it's not mass market and the setup may be harder ( it's years since I had to deal with leased line so things may have changed ).
One advantage of a leased line is that it is symetric, so you can go as fast up as you can down, but for trading I don't see any advantage in it.
I think you need to ask yourself what's wrong with broadband, and how leased line would solve that before paying the extra $$$$.
dave