Hmm... when it comes to the details of a sound things get complicated.But they are details.About the grain..yes, you can barely hear the difference in tone.Personally i wouldnt examine this little detail that much,but as hairbnd mentioned the type of the wood matters the most.When you get to actually trying to make a nice sound you will see that this little detail isnt noticable anymore.For example, when i try to make a nice full blown solo sound there are so many things that i add to to the sound so that even the type of wood doesnt matter anymore.Pickups yes,they matter.Type of distortion yes,it matters.Type of compression yes it matters.Neck pickup or bridge pickup yes,it matters.But not the grain.Not even the wood.
If you wanna play clean,yes the wood matters.It makes a big difference.But not the grain.
The only thing that you must do to be completely sure what guitar to buy is to go to a store and try both the gibson and the epiphone very very inspective.Play with both of them using the same settings on the same amp and try to hear which of both you like best.Or even better,if you have a rather small amp,get your amp with you at the store and try them on it.When i wanted to buy cabinets i used to drag with me my preamp unit and test the cabints with my preamp.It is the only way to make sure that you get the tone you like.