<i>could you compare the 512kbs to 1.5megabites... </i>
one is bigger

basically 512 vs 1500 - a factor of 3
theoretical transfer rates, getting 80% - 90% of your advertised rate is considered good as there's always local, line conditions, and 'overhead'
generally agree re cable vs dsl comments above but it depends on your cable and telco/dsl provider(s) (i don't agree with the 'cable is shared' argument that telco advertising use, dsl networks become shared at a certain point before the internet also - but that's a different discussion)
i have a 4500 cable connection, even at it's slowest it doesn't fall below 2000, but it's usually 3200 to 3800 throughout the day and night. (and i also distribute dsl flyers around the neighborhood so people stay off my cable line

)
in your case - sounds like dsl may be a better connection speed wise but remember:
price differences
cost re buy or rent modem
phone line hassles sometimes
here's a
bellsouth user forum with some good links
and there may be other ISPs that service your area, using Bellsouth lines, try an
ISP search here