The Boston Celtics have a longer more storied history than the Oklahoma City Thunder (formerly the Seattle Super Sonics). The Celtics also have the best young brain in the game as Head Coach in Brad Stevens (he's just 39). He's overseen the rebuilding of the Celtics franchise post "Big 3" (Garnett, Pierce, Allen) and he's taken the team from the cellar 25-57 in the '13-'14 season to the playoffs the very next season with a 40-42 record and they went to the playoffs again with a record of 48-34 this season and all this without a real superstar on the team.
As for defending a SG vs a SF it depends on the kinds of players and style of play a coach uses. Some coaches like to play the traditional 1 Center, 2 forwards, 2 guards other coaches opt to go 2 forwards and 3 guards and it depends on what kind of talent you have on the team. Kevin Durant is tall, he's 6'9 but he's not muscular enough to play in the post he's what you would call a "Swingman" not unlike a football player who could play both on the Wing or as a Striker and that is used to the team's benefit flip flopping those roles during the game to try and get better defensive matchups. The ultimate differences in small forwards and shooting guards is rebounding and perimeter defense.
Notable Small Forwards: James Worthy, LeBron James, Kevin Durant, Scottie Pippen, Paul George
Small Forwards also tend to be taller than shooting guards, but it's not necessarily a given for that to be the case.
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