Call to Arms is for returning players, that lasts only
10 days but it includes
all Warhammer content. You can opt out of this offer by the 9th or 10th day if you don't want to subscribe right after checking out the game.
Endless Trial never ends but is
only the Tier 1 content with restrictions. The ORVR and Scenarios victories still go towards helping subscribed players in T2 with zone flips and the like.
So you can try the Free Trial (on a separate account), try out the Knight/Slayer/whathaveyou if you haven't already (enhanced with the new tutorial/changed starting zone which isn't cluttered
) and then hop on the Call To Arms 10 day offer (on your normal account) to check how the endgame is when you have the free time.
I personally checked out the game first on the Endless Trial, liked the improvements, came back during a break in schoolwork, played my T3 Engi for a week and made a Slayer, loved the changes even more, and now I'm subbed since finals are done.
And for the record, Order is usually the victor in T2 and T4. Expect to see a lot of "Order just pwned Destro city" in chat.