SST AL isnt much lighter than a Kilo TT would be from my experience. I saw some Kilo TT's today and was very impressed by them.
(There is a guy local who is selling a SST AL and says it weighs in at alittle over 21lbs, which is heavy for alum.(Also cant comment on how right/wrong he is.))
I would say go for the Kilo TT , im betting that alot of others will say the same.
Regarding the size if you have a 31-32" inseam, i would go for the 50cm if your 5'9". Dont know how long your arms might be but i think it would be best to stay on the smaller side since your 5'9".