The train would be quite a bit faster--as little as 3 hours vs. 4 hours or longer on a bus, if you were simply traveling from downtown Barcelona. However, the bus company ALSA has a few direct buses from BCN airport to Valencia that take as little as 3 hr. 50 min. and avoid the need to get to the train station in downtown Barcelona and switch to an express train. Not having to make that connection in Barcelona might make up most or even all of the difference in travel time. Those BCN-to-Valencia buses are not cheap, however; don't expect them to be substantially less expensive than the express trains. The bus fare for February 18 is about 34 euros.
The ALSA buses I've taken (no experience with this route) have been very comfortable. If you happen to be traveling with non-carry-on luggage, it may be easier to manage on a bus; you should be able to store it under the bus.
In Valencia the bus station isn't particularly close to the Joaquin Sorolla train station used by most of the fast trains. The location of your hotel could make a difference here if you don't like to spend money on taxis.
Neither buses nor trains are super-frequent between Barcelona and Valencia, so this decision might come down to which has a schedule more desirable for you.