I've got both concentrates and flowtoys- I've not had either for long so can't offer any comments on longevity etc, but, in terms of recent spinning-
both are good- very nice colours and easy to scroll through the modes.
Flowlights come in fixed colours (e.g. blue/white) but have 12/13 different modes, with a variety of strobe effects.
Concentrates have a variety of different colours, mixes and strobes.
Both are bright, the concentrates are brightest.
Big difference is the weight- flowlights are almost as light as glowsticks- which I've really enjoyed and it's got me quite inspired into doing some really fast spinning incorporating a few glowstring-style wraps.
(Incidently, IMO, getting the protective cases is
vital for the flowlights, I think mine wouldn't have made it this far without them

the cases also diffuse the light and, IMO, look better than without the cases).
Concentrates are heavier- very sturdy build, a bit like the old aerotechs, but not quite as heavy.
Flowlights are considerably cheaper (with any electric stuff, you need to be aware that malfunctions are quite likely, so it's best to buy at least one extra (preferably two) piece and, this is obviously easier with cheaper toys).
Flowlights are very small/light- easy to slip into your bag or pocket and have at all times ready for a spin.
One niggle about the concentrates is that they've got the battery pack on the outer end of the casing- personally I don't like having that 2-3" of wasted space, that limits closeness in, for example, buzzsaw moves- I'd prefer it visually if the battery pack was on the other end.
(Flowlights light is diffused along the entire length of the head)
Theoretically the concentrates should be longer-lasting, as they're built like tanks relative the the flowlights- however, my experience of electric kit is that, however well it's covered by plastic/silicon, sometimes it just dies (aerotechs for example).
Concentrates control button is covered by the silicon casing- good for sturdiness, but it does require a very focused push action to change the modes- those without strong fingers may find it a challenge- on the good side, they do have a 'learning mode': I've yet to use this, but it promises that you can put your favourite strobes etc and the beginning of the sequence- that kind of customisation could ensure that the stiff button isn't a problem.
Flowlight acknowledges that removing the battery case lid is somewhat difficult- however, having used mine I actually like the design- it's very innovative and should ensure that accidental pop-offs don't happen: When it comes to battery changing I'd say it's
vital that you read the instructions that come with the flowlights and, if you follow them, you'll have no problems- if you just try and work it out yourself, you'll possibly damage them.
Personally, I think probably
all spinners should have a set of flowlights- they're so cheap and look so good that there's no reason not to- certainly I find them much nicer than most of the glowball ended poi around.
Concentrates are for anyone who prefers a heavier/sturdier set of poi, or for those who already have flowlights and want an additional palet/other colours.
(Once again I'll point out that I've had neither very long, so cannot comment at all on the longevity of either product).