What similarities and differences are there between Yemenite prayer books (ie: Siyah Yerushalaiyim or Torath Avoth) and Torah Judaim's Ways of Torah Siddur?
The similarlity of these siddurs is limited to their shared influence from Maimonides' text of prayer as found in Mishne Torah and their accord with the laws of prayer in Mishne Torah. The wording of prayers in the traditional Yemenite prayer book, however, is much more in line with the wording of prayers found in Mishne Torah than that of the Ways of Torah Siddur. While the Ways of Torah Siddur is more influenced by Maimonides' text of prayer than non-Yemenite siddurs, it is also influenced by the wording of prayers found in Saadia Gaon's siddur and that found in the traditional Ashkenazi siddur. This is not the case with the traditional Yemenite prayer book, the Tiklal.
Apart from some similarity with the traditional Yemenite prayer book, the Ways of Torah Siddur is very different. As with virtually all other siddurs available for purchase, the bulk of Yemenite siddurs consists not of obligatory prayers, but of page upon page of unobligatory traditional prayers that developed in Yemen or were adopted by the Yemenite Jews from other Jewish communities at different points in history. There is nothing wrong with unobligatory traditional prayers, but unfortunately, even most people who grow up Torah observant have little idea as to what is incumbent upon them from Talmudic law and what is later tradition. Consequently, when pressed for time, people try to cover all the traditionally recited text rather than focusing only on what is obligatory. The result is that people end up having very little if any intention, whether when reciting the obligatory or the unobligatory prayers. And sadly, for many working individuals, this is a daily occurance; naturally so, because they're always pressed for time. Just as R' Avraham ben ha-Rambam described in his book Ha-Maspiq, the assumed additional requirements for Jewish prayer have given too many minyanim the appearance of a joke, not that anyone's laughing. The most critical aspect of prayer is intention. We must first have a reasonable level of intention while reciting those prayers required by the Sanhedrin before we accept upon ourselves the burden of extra recitations.
What is possibly a greater negative outcome to the assumped formate of prayer is the impression it gives to outsiders when they see the scene of Jews flying through their prayers, occasionally glancing at their watches or looking around at wherever else as their lips speedily fly beyond 100 mph. Not only does this not sanctify the Name, but it desecrates the Name by giving the impression that one's relationship with the Creator must be relegated to mindless lip service if one desires to keep Torah. Most people don't learn in yeshiva or kollel where more time is allotted for prayers. As a solution, the Ways of Torah Siddur includes only those prayers that are obligatory by Talmudic law with the appearance of an unobligatory recitation only when it is mentioned in Mishne Torah as a universal custom. The few appearances of unrequired recitations are clearly specified as unobligatory in the text of the siddur.
Beyond just the format of prayer, the wording of the Talmudicly required prayers in the Ways of Torah Siddur is also unique. In harmony with our focus on increasing intention in prayer, we have invested the research necessary to condense the wording of the required portions of prayer to the bare minimum possible without innovation. This was achieved by taking the major traditions of Jewish prayer into consideration, including only those wordings common to them all. Our goal is to greater enable the masses in the upholding Torah and in drawing near to God. We hope that the Ways of Torah Siddur will serve as an aid to that end.