In 1991, yes - on balance the Williams was possibly the better car. Certainly in the middle part of the season. But the start was tricky: Mansell didn't finish a race until round four and didn't qualify on the front row before Montreal (round five). Every race he finished he finished either first or second, but he was 34 points adrift of Senna already by round 4 (the equivalent of being 93 points behind using today's system), so the head start for Senna/McLaren was massive.
Also; Patrese (in what may have been his single most impressive season) only outscored Berger by 10 points, and McLaren won the constructors title quite comfortably so it's certainly a debatable topic.
Senna was on another planet for the US GP.
For Brazil, you get the impression Williams were quicker in the race, though Mansell spun and Patrese really should have beaten Senna who was stuck in gear.
Mansell was taken out by Brundle at Imola so we don't know what would happened to him though he did have gearbox issues. Patrese was driving at a pace that Senna couldn't match, but more reliability issues so the McLaren was the car to have.
Monaco was another Senna job, but let's give it to McLaren.
Williams were dominant in Mexico, Canada, France, Britain and Germany.
McLaren had a light weight car for Senna in Hungary though Mansell was hounding him until his tyres dropped off. Hard to know how hard Senna was pushing.
Belgium, Italy, Portugal and Spain clearly belonged to Williams.Granted, Mansell retired from Spa with mechanical issues.
Japan is tricky as Senna was intentionally holding up Mansell to allow Berger to get away but I think McLaren seemed to have the edge at Suzuka judging by the pace of Patrese.
Adelaide was the farce that it was.
With the Honda qualifying engines, it at times allowed McLaren to have track position but in terms of a competitive race car, I would opt for the Williams by some margin. I would have taken the Ferrari over the McLaren for Canada, France, Britain and Germany too.