Obi-Wan Kenobi is one of the most beloved characters in the Star Wars franchise. He first appeared in 1977 with Sir Alec Guinness at the reigns. Guinness introduced the character (and the Jedi) to fans everywhere and became the great and wise sage that would eventually become an iconic figure.
In 1999, Obi-Wan made a return to the big screens as Ewan McGregor portrayed the young Jedi training under Qui-Gon Jinn. Though Guinness set the stage for Old Ben, it seems as though McGregor paved the way there as many fans completely fell in love with his performance and consider him to be the best part of the prequel trilogy.
Similarities: Wisdom
It is no question that Sir Alec Guinness plays the wise sage version of Obi-Wan Kenobi, as he is a seasoned Jedi, hiding from the world as he watches over Luke Skywalker. However, it could be argued that Ewan McGregor's Obi-Wan was also wise beyond his years. He seemed to always know what was going on behind the curtain. He knew that Anakin and Padmé shared deeper relations than they let on, without revealing it to the council. Kenobi knew that something just wasn't quite right with Palpatine or Dooku and just always seemed like he knew something was wrong before everyone else did.
Differences: Humor
While both iterations of Obi-Wan, had a bit of humor going on their styles were vastly different from one another. Sir Alec Guinness seemed to be a bit more straight forward when it came to things. He kind of had the "kooky old man" thing going for him and could be seen rolling his eyes at Han Solo and his brashness. But McGregor was a whole other story. He would say amazing one-liners and quips back at Anakin or the other Jedi (and some of the bad guys) that made him truly iconic and hilarious. He was sarcastic and dry and really didn't like flying.
Similarity: Love
Though the character would probably never admit it, Obi-Wan has a ton of love for those close to him. He cares deeply for the Skywalker family as is made evident by his watching over Luke and care for Anakin.
There really is no debate about Obi-Wan's love for Anakin, but since fans had such a short time with Guinness, it might not be as apparent. But he did help Luke, and watch over him his entire life, trained him to be a Jedi, and even stayed with him after death.
Differences: Fighting Style
The fighting styles between Ben and Obi-Wan are kind of a big one, as there is a very definitive moment for Obi-Wan when he transitions to the new that isn't seen in the movies. In the prequel trilogy, Obi-Wan is a very defensive fighter and uses what is known as "Soresu," which is where he gets his signature pose from. Soresu was made by the Jedi Order during the rise of blasters. It is a fighting skill used to reflect the blaster bolts and always be on guard to defend. Old Ben uses this too but changes it up a bit by incorporating some of Qui-Gon Jinn's moves, which is "Ataru," which he uses for his final confrontation with Maul in Rebels and then again with Vader in A New Hope.
Similarities: Style
Though Jedi robes seemingly went out of fashion 20 years prior to Luke discovering Old Ben on Tattooine, that didn't stop the Jedi from wearing his favorite garb. McGregor's Obi-Wan was obviously based on Guinness's, and the two seemingly shared their interest in loose, earth-toned robes and billowing cloaks. Even the hairstyles resemble each other (especially Revenge of the Sith and A New Hope). Those 19 years really did do a number on Old Ben, however.
Differences: Doubt
Of course, Obi-Wan trusted Anakin more than anyone he knew. Anakin was like a brother to Obi-Wan and it was apparent he cared for him more than words could convey. However, he often doubted the younger Jedi whenever Anakin would do something a little extreme. He would never fully trust that Anakin's plans would work but just hoped that they would. Guinness's Obi-Wan did trust Luke, and though the young farm boy didn't try anything remotely as dangerous as his father, Ben still had faith in him as a Jedi.
Similarities: Represents The Jedi
It seems like in both, the prequel and the original trilogy, Obi-Wan Kenobi was the epitome of what it meant to be a Jedi. In the OT, he the first example we get of the space wizards, as he introduces to fans what the Jedi even as he whisks Luke off onto his new adventure.
This concept is a little more muddled in the prequel series, a trilogy that is plagued with Jedi filled with corruption. But Obi-Wan remains the symbol of what the Jedi originally stood for. He isn't all about the war, like Mace Windu, nor does he scorn Anakin for having affections. The Jedi Council was plagued by their own rules and seemingly forgot what it actually meant to be a Jedi, something that Obi-Wan manages to portray perfectly.
Differences: Charm
While Sir Alec Guinness definitely has a certain charm about him when it comes to his role as Obi-Wan, McGregor takes Obi-Wan's charm to an entirely new level. The way he interacts with pretty much everyone he meets, even his foes, is always done with such simple grace. His sarcasm and sass always manages to come off as more charming than anything else. Even the way he says "Hello there" is swoon-worthy.
Similarities: Sass
One of the key characteristics about Obi-Wan Kenobi is definitely his sass. In both iterations of the Jedi, Obi-Wan is incredibly snarky and sassy, no matter how refined he likes to pretend he is. There is no one safe from the attitude this character portrays, not Anakin, Luke, Han Solo, or General Grievous. While McGregor definitely plays it up compared to Guinness, it is very apparent that he was inspired by all of Old Ben's quips.
Differences: Mysterious Wizard Vs Snarky War General
McGregor manages to pretty much capture Guinness's essence through his time during the prequel films. The old sage laid down the foundations for the future of the character, while the young Jedi manages to build the rest of the house that would lead to him. But one of the biggest differences between the two isn't the way they act or present themselves, but rather where Obi-Wan actually is in life. Guinness played the classic mysterious wizard, the definition of "The Mentor" in Joseph Campbell's Hero's Journey. But McGregor is Obi-Wan during a war-stricken time. He comes in at the openings of a war against the Republic and the Separatists, becomes a general in a political war (something the Jedi never should have been), and is forced into these dark times by the state of the galaxy. The difference between the wizard and general is pretty extreme but also shows the character's growth wonderfully.