Pine = soft wood, Oak = hard wood. It really is that simple 
various environmental conditions, including how the wood was cured and the integrity of the tree from which it was cut, affect how long boards last and how straight they stay.
SYP is indeed better than white pine - there’s a good reason SYP logs are used for houses over white pine
Fast growing trees, including poplar, are also indeed less dense, making them less structurally sound than slower growing soft woods and hard woods.
Pine
I have a paddock that was fenced in 1991 with rough pine boards and PT pine 4x4’s. They’re still doing the job some 21 years later with only one coat of stain and electric fence to keep the horses from chewing it.
Pine is cheaper so that’s what I use.
OAK
Just to clarify, when I was talking about pine I was referring to Southern Yellow Pine as well.
This has been very interesting. I wonder what the OP went with?
Definately a regional decision - we use cyprus here - bug resistant and OK for wet spots too. Posts are 24 years and still going (for the most part), and that is thinner posts.
Oak–they far outlasted (by many years) the few poplar that got mixed in with the truckload we ordered and much less prone to being chewed up.
Planning for your plank fence is quite tricky because there will be a lot of suggestion about how to do it and what materials to use for it.
You indeed have to determine first the site - the dimensions of the area, and the perimeter of the fence. But when it comes to deciding what type of wood to use, you have to consider not just the aesthetic value but the quality and rot resistance it can give; of course you want the best that fits within the budget as well. I personally think oak is a better choice than poplar in the factors mentioned above.
Oak should be white oak, not red oak, for outdoor applications…
I know many folks use Tulip Poplar for fencing and it’s generally inexpensive, but honestly, it’s not a good species for outdoor longevity.
Oak
This thread is from 2012. Hopefully OP has made up their mind by now!
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