I read the message, I understood it too; still doesn't mean that it should be that way...
you spend precious time making a character, and yet the game wants you to make an unchangable decision based only on a brief description... That isn't right. If you like the sound of one over the other, suddenly you could have a character that you don't like anymore.
The game should either allow players to test out the two specs before hand, or allow you change specs at a cost or certain amount of times.
And yes, they are specs not different classes. Classes are what you choose from the begining, you get a feel for them in only a few hours. If you don't like how it feels, then you roll a new character with little to no loss. But for these "advanced classes", you have to gamble on loosing a character that you have spent valuable time and resources into over a single question that you have nothing to base your answer on... No other MMO with advanced classes does this; and yes there are other MMO's that have advanced classes (Final Fantasy online and Ragnarok online are two examples, and both allow you to respec to be different adanced classes).
In ToR you have a Sith Warrior, just as Ragnarok has a Swordsman... The Sith Warrior can become a Sith Marauder or a Sith Juggernaut, just as the Swordsman can become a Knight or a Crusader... Then the Sith Marauder can become a Annihalation Marauder or a Carnage Marauder, just as the Knight can become either a Sword Knight or a Spear Knight... And so on... So... Two different games, each with multiple "advanced classes", the difference? Ragnarok allows you to respec to a different (but equal) "advanced class", when you decide the one you have isn't for you, instead of wasting your time having you create a completely new character.
Time is money, doubley true for a game that you have to pay to play. So the more a game makes you feel like you've wasted your time, the less you'll want to play it. This makes the "absolutely no changing advanced class" idea a pour design choice in the long run for Lucasarts/Bioware/EA.