"Sport" means what/ Less expensive and less comfortable?

It means less expensive and heavier.

I think the sport has Mg rails. I have what is known as the Alliante "Gamma"; mine has Ti rails and a cutout under the padding. The gamma is made with a less expensive shell material (some sort of graphite cloth weave.) The top end Alliante has either Carbon or Ti rails with a carbon shell.I do not know the shell construction of the "sport", but maybe it is the "Gamma" w/ Mg rails(heavier) I have no idea of the comfort comparison, maybe you can call a shop such as Excel and ask them what they think.

I think the sport has Mg rails. I have what is known as the Alliante "Gamma"; mine has Ti rails and a cutout under the padding. The gamma is made with a less expensive shell material (some sort of graphite cloth weave.) The top end Alliante has either Carbon or Ti rails with a carbon shell.I do not know the shell construction of the "sport", but maybe it is the "Gamma" w/ Mg rails(heavier) I have no idea of the comfort comparison, maybe you can call a shop such as Excel and ask them what they think.

hmm... I bought a used stock giant tcr composite 2 and put my own (campy) components on it. the stock saddle was an aliante sport, and the rails said that they were ti. it did not look as well constructed as the top end aliante, but it looked good enough. I never sat on it, because SLRs fit my rear better.