Here are some side by sides of a Mantis size M and a Flex size 1. As for weight, almost double, however, keep in mind that the M Mantis weighs almost nothing, so the Flex is not heavy by any means. Also a L Mantis is probably closer in size to the Flex 1.
Every type of webbing used on the Flex seems to be a bit more heavy duty and rugged. Dont get me wrong, i have no doubt the Mantis is fully capable and is beyond safe enough for what we are doing. Just stating an observation. Also the mantis has 1 waist buckle, which is the same as the Flex's 2 leg buckles, and the flex has a larger waist buckle. I feel this is where a fair amount of the added weight comes from.
I really like both. Not much difference leaning, Flex has edge sitting. Really like LM loops on Flex. Flex also has more easily usable mollee webbing i feel. Obviously the adjustable webbing bridge is nice on the Flex, especially for a noobie. I saw the elastic webbing others have sewn into flex portion of the Flex and this a great idea/improvement. Also i think that mesh material like the Mantis or Kite would make for a better saddle in my opinion. But not sure if you could do that with flex feature or not.
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