We have pretty much got through the past (at least) 15 years of mountain bikes with seatposts and seat tubes causing no problems at all, its a sad day when we have to start pointing the finger at a seat tube and seat clamp design when we know that the problem stems from the wall thickness of some new fangled seatpost that hasnt yet been perfected.
and by that i mean the internals made small enough to enable a wall thickness that can do the job like all the stuff we have got on with fine over the years...and thats even before they get the reliability sorted on these things. Start from the outside in!! We have used the likes of PUSH posts retailing around £20 and even at that price,they do not fail because of a wall thickness thats too thin.
The only problems that we have (and its not even a freekin mtb) is on mountain bike style bikes such as real poor quality stuff like Universals,Emmelles etc. Those chrome plated steel posts that get pinch marks before the seatpost is securely fixed. This is mainly on mtb style bikes costing around £70-£130. Lets not go there.
Its about paying £150-£300 for a seatpost that now has possible problems with its wall thickness,not to mention the internal reliability.
If this is the case,any brands that cant make them strong enough,stick with 31.6 and forget 30.9 until you nail it on the head with decent testing.. instead of blurting these things out for £270 a pop,using the public as guinea pigs..
yet again ;O)