

Assalaam Alaykum,
I have a question regarding the makhaarij and sifat of the taa. I heard some people say taa when it has a vowel it should not have any hems, this sounds like the Urdu taa. Others say taa always (when vowelled and non-vowelled) has a hems, meaning it always has this sharp breathy sound which kind of sounds like the English "T". Which way is correct?

Wa alaikum assalam wa rahmatullahi wa barakaatuh
The inherent characteristics of the letters are present in the letters are all times, whether the letters are saakinah or voweled.  The only exception which some may give is the qalqalah, for those who count it as an inherent characteristic and present only when the group of qalqalah letters are saakin. 
For the group of opposite characteristics, such as and its opposite , the characteristic never leave the letter, but it is more audible or discernible when the letter is saakin.  For the letter we have to be aware of two possible mistakes when it is voweled:  1.  not giving it any of its inherent characteristic of , and the other, of our tongue hitting our teeth to pronounce it and the resultant sound is one of excess which is similar to the whistle sound of the letter
The English  "T" is not the same as the Arabic , and the English T (in most cases) has excessive air in it compared to the Arabic .  We need to make sure we use the top of the tip of the tongue when pronouncing the , and let it hit at the area where the roots of the teeth are and not further back on the roof of the mouth, and also not on the teeth.  The sound of hams is not to be over pronounced, nor is it to be imprisoned. So the hams is heard when voweled, but not excessively.
 The Urdu ta (there are two Urdu ta's from our understanding) is pronounced back further in the mouth.