I have never seen a siddur that has TOras. The correct pronunciation, as far as I know, is toRAS.
In general, the only reason a mil'ra word, such as toRAS, would become mil'ail, is if the word following it had its accent on its first syllable. For example, if the term was "toras chesed", it would be "TOras CHEsed", as the accent in "chesed" is on the first syllable.
This rule is called "nasog achor". [Note: This rule only applies if the word following it is "connected" to the first, i.e. the two words form a phrase, like is the case with "toras chayim" and "toras chesed". The signifying factor for this in Tanach would be if the ta'am (trop) of the first word was of the "connecting" type.] See Mishlei 31:26 and Tehillim 119:72.
Here, however, the following word is "chayim", which is mil'ra. Therefore, it remains "toRAS chaYIM".