i think OKC is the better team and over 100 games probably wins 55 of them. i think Miami at their peak is better than OKC at their peak because of Lebron/Wade/Bosh. The home court advantage clearly will help OKC as well.
ultimately it comes down to whether Miami can get help when Lebron or Wade has an off night. OKC has a bigger buffer if Durant or Westbrook is off with Harden/Ibaka/Perkins there to lift the slack.
Turnovers will be a big factor for the Thunder. they are a better offensive team than Miami, but Miami is a better defensive team... the turnovers will ultimately make the difference in my opinion.
Is LeBron/Wade/Bosh definitely better than Durant/Westbrook/Harden?
Slight edge LeBron over Durant, Slight edge healthy Wade over Westbrook (but I think Wade isn't healthy and thefore say Westbrook is more valuable over 5-7 games), then Harden is more explosive than Bosh, Bosh might give you more on defense/rebounding side. So its about even.
Going to full team analysis including Bench:
Westbrook v Chalmers - Big Adv OKC
Wade v Sefolosha - Adv Miami
LeBron v Durant - Slight Advantage Miami
Bosh vs. Ibaka - even
Perkins vs. Haslem - even
Bench Harden/Collison/Fischer vs Battier/Miller/Jones - Big Adv OKC
Also worth noting that OKC's big front line makes Miami play Haslem&Bosh together a lot to counter the size of Perkins/Ibaka/Durant.
So UNLESS Durant totally cannot stop LeBron and is stopped by LeBron I think OKC has more significant edges and can neutralize Miami's strengths (Sefolosha on Wade, Ibaka on Bosh, Durant's scoring offsetting LeBron's). OKC should dominate the glass and it'll be really interesting and fun if both teams run. Could have a 120-118 type game?!
I really think the series might come down to the marquee matchup though and it seems to me LeBron's chances to keep Durant in check are better as Durant's only advantage is length on outside shots. If you are Miami you want Durant hoisting long jumpers. Meanwhile if you are OKC I think you are nervous about LeBron overpowering Durant and wondering if Durant & Westbrook are ready for the big stage.
Team-wise I think OKC is better and could dominate, but intangibles go to Miami. LeBron is better than Durant and he absolutely knows his legacy will be recalibrated if he wins that matchup and his team wins. So I think the LeBron from Game 6 vs. Boston shows up and the Heat win in 6.