I recommend you to choose EIC. TLK have lover standard and having no reserved coach is ridiculous.
TLK doesn't necessarily have lower standards - while the name tells you what you need to know (it's meant to be cheap!) - they frequently use EIC carriages on TLK services.
What's ridiculous about unreserved seating? It costs money to administer reservations, and the lack of reserved seating means that boarding times are faster. If you want a reservation, you travel 1st class - simple.
Anyway, from Poznan to Gdansk, the journey sucks regardless of which type of train you use. There's no direct EIC service, so you have a choice of InterRegio or TLK services. For such a long trip, I'd steer clear of InterRegio (though it is cheaper!) due to the unreliable quality of the services.
As I recall, Poznan-Gdansk is through Bydgoszcz.