Just a comparison of two whistles I own so if you want one you know their faults.
I've heard that the whistle used by Klarus, Titaner and Versati are all the same, with the exception of each baring a different logo. Therefore my Klarus whistle will represent these three whistles in the comparison.
Length: Both whistles are exactly the same length. However in width the 4sevens is longer diagonally (by 1mm), but when flat it is smaller than the Klarus (by 1mm).
Keyring Attachment: The Klarus whistle is far easier to attach a ring to. The 4sevens is more tricky, I used some pliers to attach the ring to the 4sevens. Also the Klarus can accept smaller rings than the 4sevens - the ring I am using for the 4sevens is the smallest that could fit on it, and it is a lot larger than the Klarus.
Strength: I'm going to guess here that the 4sevens is lot weaker than the Klarus because the walls are very thin in some places. I can imagine those places collapsing when put under pressure. The Klarus has consistent wall strength/thickness.
Ergonomics: The Klarus wins here again. The mouth piece shrinks in the middle then grows out - this lets you grip it with your mouth more easily. The 4sevens does not have this.
I don't think the square shape of the 4sevens is as comfy as the round Klarus but I think this shape make it more easy to grip in hand than the Klarus.
Sound: The 4sevens wins here. It takes a lot less effort to make sound than the Klarus (I'm guessing 5%) and I think a bit louder too but not by much.
Conclusion: If I could buy all over again, I'd probably get the 4sevens only and save my money. Sound is more important than any other factor to me, though if ergonomics or strength is more important go with the Klarus.
BTW can anyone tell me how long should repairs take? I've been waiting 3months + 17days for a repair so far...