Where they differ
Strong is built around logging. You pick or build a routine, then use the app to record sets, reps, weight, and rest timers, with a plate calculator and strong progress charts for lifts over time. It does not generate your program or tell you what to do next — you decide the plan, Strong helps you execute and track it.
FlexFit works the other way around. You tell it your equipment, schedule, and how sessions have been feeling, and it generates the next workout and adjusts week to week based on what you logged last time. You can still run things by hand — build your own program, swap exercises, add or drop sets — the difference is FlexFit will make the programming decisions whenever you don't want to.
The other big gap is scope. Strong is focused on lifting logs. FlexFit also includes mobility routines, food and macro tracking (by typing or photo), and body weight tracking alongside training, so your training and your body-weight trend live in one app instead of two or three.
Platform-wise, Strong is a native iOS and Android app, which matters if you want offline reliability and a very polished native feel. FlexFit is a mobile-first web app you add to your home screen, with a native iOS app on the way, if a native app matters to you today.
Who should pick Strong
Pick Strong if you already run your own program — your own split, your own progression scheme — and just want a fast, reliable place to log sets and see your lift history and personal records over time. If you want a deliberately manual-only tool with nothing else in it, Strong is a polished one.
Who should pick FlexFit
Pick FlexFit if you want the programming decisions handled for you: what to do today, how much to add, when to back off, based on your last session and how it felt. It's also the better pick if you want food logging and body weight tracked next to your training instead of in a separate app. Beginners get a guided setup with sensible defaults; experienced lifters can set their own split and volume preferences and let FlexFit manage the week-to-week adjustments from there.
Which one for you
If all you need is a logging tool, Strong is a good one. FlexFit lets you design your own program too — and adds generation, macros, and body weight tracking when you want them — so the real question is how much you want living in one app. Both let you try before committing — FlexFit has a 7-day free trial.