Answered with numbers you can check — including ours.
Flat staking puts the same amount on every bet. Kelly sizes each bet by the size of the edge. Both are defensible, and they answer different questions.
Flat staking is more robust when you are not confident in your probability estimates - which is most of the time. It cannot be gamed, it is easy to audit, and it makes records comparable between services. Its cost is that it under-bets your strongest opportunities.
Kelly grows a bankroll faster when your probabilities are well calibrated, and protects it by shrinking stakes when the edge is thin. Its danger is asymmetric: overestimating your edge, which is extremely common, leads to overbetting and severe drawdowns. Fractional Kelly - a quarter or a half - or a hard stake cap removes most of that risk at little cost to growth.
For reporting, flat staking should always be used. A record shown at Kelly stakes cannot be compared against anything, because the sizing decisions are part of the claimed result. MalluSports sizes with a capped Kelly but reports ROI flat, for that reason.
MalluSports is a free prediction and analysis service only. It accepts no bets, holds no funds, operates no gambling platform and holds no gambling licence. Betting carries real financial risk. 18+ · begambleaware.org