Light meter

You didn't say which model you are considering but the ones I looked at are LUX light meters so any conversion to PPFD will have to be done manually. The one benefit is they might have more accurate calibration out of the box but without knowing what part of the spectrum it registers it is hard to know if it will be accurate with your lighting. If going less expensive I also use "PPFD Meter" from the Google Play store. Every app needs calibration to make it accurate with your lights but even if you don't it's repeatability is good. And, if you do calibrate to your light then it can be quite accurate and usually within 10% and I think much of the variance comes from a dirty glass over the phone's sensors.