If you're new to the game and the forums you ought to check out our
New Player Center.
Make sure to check out the
professions page as well. Read up on each character's primary attribute at what it does.
Finally, while it's true that a Mesmer/Warrior has access to all the same skills that a Warrior/Mesmer has, they will have completely different playstyles.
For example, a Mesmer/Warrior has access to Fast Casting, the mesmer primary-only attribute. That also gives him access to Mantra of Recovery, a skill linked to Fast Casting that makes all spells recharge twice as fast. He will still be able to learn skills from Strength, the warrior-primary only attribute, but he wont be able to improve his strength at all. So while he could learn power attack, he will never be able to improve the damage his power attack does, making it essentially a useless skill for him.
Similarly, a Warrior/Memser will likely use a skill like power attack regularly, but Mantra of Recovery will be useless to him, because it requires an additional energy cost with each spell cast, and the cost scales with how high your Fast Casting score is. The Warrior character will have his fast casting permanently set to 0- so using the skill just isn't realistic at all.