Both the light and dark side of the force are capable of being used to do incredible things; Yoda pulling the X-Wing out of the swamp or the Emperor's use of lightning from his fingertipsto inflict pain. But it's clear, from the evidence gathered from the six Star Wars films, that the dark side is the more powerful side of the force. This superior trait can be seen not just in lightsaber battles but also in the more subtle use of the force to confuse and mislead foes.
For example, a single Sith lord was able to masquerade as a seeming leader of the Galactic Senate, masking his intentions and clouding portions of the force from the most powerful Jedi in the galaxy for years. Darth Sidious, also known as Emperor Palpatine, engineered an incredible ruse whereby he became Chancellor, instigated a war, and managed to keep his identity as a powerful Sith lord from the legions of Jedis that surrounded him secret long enough to complete his coup d'etat over the entire galaxy. He somehow turned the vast majority of the Galactic Republic's military forces against the Jedi, effectively defeating the Jedi as an organized sect and ending the war with the Trade Federation virtually overnight. As well, Sidious managed to manipulate and exploit one of the most powerful wielders of the force to be seen in centuries when he turned Anakin Skywalker against his former brothers-in-arms, especially his mentor, Obi-Wan Kenobi. Such feats can only be attributed to an incredible use of the dark side of the force, whose power was demonstrated by how long and total the charade of Palpatine as an ally endured.
Darth Sidious wasn't just powerful in the use of the dark side for manipulation, however. He also, after being confronted as to his true identity, easily killed several Jedi Knights in mere seconds with the overwhelming power of the dark side. It is unsure whether or not his failure to also dispose of Mace Windu in single-combat was due to the Jedi's own prowess or if Palpatine was simply delaying the fight until he knew Anakin would be there to finally complete his betrayal.
In any case, there are other examples of Sith taking on multiple Jedi and barely breaking a sweat. Darth Tyranus fended off the challenge of Obi-Wan Kenobi and Anakin Skywalker at the same time, crippling Anakin and disabling Kenobi rather easily. After this battle, he fought Yoda, the greatest, most powerful Jedi, to a standstill. In the next film, Tyranus again fights two Jedi at the same time, and is only defeated when the more powerful Anakin (who wields two lightsabers) is manipulated into embracing his anger and exploiting the arrogance of Tyranus (both traits of the dark side).
As well, in his attempt to cut off the Sith takeover before it could finish, Yoda only manages to fight Sidious to a draw, not defeat him. Yoda failed in part because he refused to tap into the dark side of the force - something which light side wielders used at other times as the only apparent way to defeat a Sith.
Darth Maul easily handled two Jedi Knights at once, and the minute he was in single-combat with one, he again seemingly kills Qui-Gon Jinn with ease. It was only when a young Obi-Wan Kenobi felt anguish and anger at his friend's death that he began to defeat Darth Maul, but the Sith's defeat had more to do with his own arrogance than Kenobi's powers of the force; instead of simply finishing off the unarmed Jedi, Maul taunts him long enough to be taken by surprise when Kenobi rises up from the pit and destroys him.
It also appears that younger, less-experienced Sith are at least equal if not more powerful than many older, more-experienced Jedi. Darth Maul had recently finished his training and he was able to defeat the much older and more-experienced Qui-Gon Jinn. Darth Vader, very shortly after embracing the dark side, fought the older and more-experienced Obi-Wan Kenobi to a standstill; it was only when his arrogance took hold that he was defeated ("You underestimate my powers" he yells to his former Master, before being crippled and terribly burned).
The reverse doesn't appear to be true for the Jedi. Younger Jedi are unable to defeat older Sith, such as when Luke Skywalker was easily handled by Darth Vader and Darth Tyranus handled Obi-Wan and Anakin with relative ease.
While it's obvious, then, that use of the dark side results in, as Darth Sidious put it, "unimaginable power", its one great weakness appears to be pride and arrogance. Darth Maul was defeated when he began taunting his opponent; Darth Tyranus was defeated when he overestimated his own skill against that of a superior opponent; Darth Vader was defeated when he again overestimated his own skill and understimated his opponent's; the Emperor was eventually defeated because he felt too superior, too confident in his own skills to sense the danger in Vader possibly betraying him (itself a characteristic of the dark side).
If one seeks power and dominance, or if one sees the need to kill an opponent in combat, one should embrace the dark side. Obi-Wan defeated Darth Maul by embracing his anger at Qui-Gon's death; Luke Skywalker defeats Darth Vader by becoming incensed with rage at the mention of his sister turning to the dark side. When the Sith are able to overcome the one great weakness of the dark side of the force - arrogance -, as Darth Sidious was able to do for decades to great effect, they become unstoppable. The Jedi's great mistake is that they refuse to have anything to do with such a potent use of the force, while the Sith's is that they don't do enough to restrict the dangers of too much power gained too quickly. While the dark side is clearly more powerful, it is not "better" in that sense; the uses and abilities of the force are there for those who choose how to use it.
