They're both "accurate." But Credit Karma uses the VantageScore scoring model, which isn't widely used. myFICO provides FICO scores. Almost all lenders use some flavor of FICO.
Credit Karma provides great TransUnion and Equifax reports, so definitely use it for that feature. However, it breaks average age of account by failing to include closed accounts in its computation. (Both FICO and VantageScore include closed accounts.)
Pretty much any score you get from any third-party (a free score that comes with your credit card, scores from myFICO, scores from Karma, from credit.com, from Credit Check Total, etc.). is accurate.
For example,if you pull your Vantage 3.0 scores from Karma for EQ and TU on April 21, then those really were your Vantage Scores based on EQ and TU data on April 21.
Likewise if you pulled your three FICO 8 Classic scores on April 26 using Credit Check Total, those really would be your FICO 8 Classic scores based on CRA data from April 26.
In other words, the third party tool is accurately giving you the score it promised to give you.
What you may mean is this: is the model that Karma uses (Vantage) more likely to be one a prospective lender or CC issuer would look at, or would they be more likely looking at one of the dozens of FICO scores you get from myFICO?
The answer is that FICO scores are more likely to be used. Which FICO score drawn on which bureau is much harder to know.
But you are likely asking something even deeper, which is: What is the best set of tools for me to monitor my credit, given my particular needs? And Credit Karma might be an important part of the solution for you -- or not. It depends on what you are trying to accomplish, how much money you like spending on this stuff, etc.
It's like asking "Which car is best?" Well, that depends....
If you tell us more about your situation, people may be able to advise you better.