Thicker denim is not necessarily more durable. The denim itself may be tougher, but the heavier the denim, the more stress it puts on the stitching, which can lead to tearing. This is especially prevalent with heavyweight, all-cotton jeans (such as Samurai's heavyweight offerings.)
In this case, Iron Heart's denim may be more durable, but not necessarily because it's heavier in weight; if you're getting a pair made from sanforized denim it will last longer because sanforization helps to reduce the abrasion that jeans undergo with wear. IH also uses poly/cotton core-spun stitching, which theoretically lasts longer than the all-cotton stitching used on Flat Head's jeans. But Flat Head's jeans are very well-made and plenty durable in their own right, especially if you don't wait ridiculous amounts of time between washes.
Rather than only looking at durability, I'd say that there are other factors you should consider. What kind of fade and texture are you looking for? Flat Head's jeans are known for high contrast vertical fading, which is quite different from the way IH jeans fade. Most IH fabrics are sanforized and have a smoother, even texture in comparison to the hairiness of new FH denim. IH's denim fades nicely, but it won't be as easy to achieve the same sort of contrast with it (the unsanforized 666 denim is more akin to Flat Head's in terms of texture/fading - but you may be getting that in exchange for the durability of the sanforized fabrics.)
There are a lot of faded FH pairs around the office here that have been washed dozens of times over the course of their use: virtually none of those have crotch blowouts or other problems with tears, etc. Overall quality is important, but at the level of Flat Head and Iron Heart it's very difficult to say that one is better than the other, it basically comes down to personal preference. If you wear Flat Head and wash them fairly often (anywhere from two weeks-two months) then crotch blowouts won't be a problem for a long time. With a sanforized Iron Heart jean, you might be able to go longer without washing. Buy the jeans with the overall feel, fade, and style that you prefer.