In my limited experience, walking the path of the "good guy" is usually more difficult in some ways and easier in others.
For example, the "good" guy is usually more socially acceptable and therefore tends to interact with NPCs (or PCs in a multiplayer RP setting) easier. He'll often be more welcome in social gatherings and is more likely to have loyal friends who will stand beside him when the going gets tough.
Conversely, the "evil" guy is more likely to have run-ins with the general populace, less likely to have people willing to help when his back is against the wall and generally can't trust his "friends," because they're probably "evil" too and will likely do whatever gives them the greatest edge rather than risk their necks for him.
The "good" guy is usually going to "gear up" slower because he's not going to take the immoral, amoral or simply unscrupulous shortcuts necessary to obtain the gear quicker. Murdering the guy that won't sell him the super-sword isn't an option. Stealing usually isn't an option for the "good" guy because it's generally not considered the moral high ground. Instead, he has to earn his gear through quests, the generosity of others or purchasing it at the listed price (with some haggling, of course!)
The "evil" guy, on the other hand, is generally going to "gear up" quicker. Murder is perfectly acceptable. Stealing? Definitely in this guy's playbook. Backstabbing employees who hold out on you? Just good business.
Taking the moral high ground has its advantages and its disadvantages just as walking the darkened path does. In the end, I've found the game experience is generally equal for both sides.
Is there a lower experience point gain for the "good" path? Maybe a little but not enough that I've ever felt underpowered or incapable of completing the game, there are plenty of alternative ways to earn experience points that don't involve my character compromising his moral compass.
Could I level up faster if I played the quest out and then murdered everyone? Absolutely! Would I enjoy the game as much? Probably not because I enjoy immersion and that would completely shatter it.
