So you are planning a trip between Warsaw and Kraków by train.
The TLK trains are cheaper than the EIC. They both take around 3 hours non-stop. Actually, EIC is much faster than TLK.
The confort at EIC is better, since seats are wider and there are two seats together. They are more clean and have air conditioner, which is good in summer.
In TLK, there is no air conditioner. At the moment, there are many works in Polish railways, so the trains make lots of stops. This makes the environment really warm in the TLK and much more unconfortable.
Anyway, TLK is good enough. It is not that bad.
You can book ahead for a Polish train regardless of your location here.