T&G floor decking--2x6 vs. 2x8

[Erin]

Post and beam framing, my centers are 3'4"

My floor decking is going to be 2x6 tongue and groove, but in shopping I find I can also get 2x8 (Southern Yellow in both cases).

Obviously I would be able to use fewer pieces of 2x8, thereby making the process a little quicker. In theory, of course.  And I like the look of the wide boards, better. 
But is there an advantage to 2x6 instead?  Since the joints are part of what give the floor it's strength, does the wider spacing mean 2x8 wouldn't be as stiff as 2x6?

[Squirl]

Shrinkage.
Lumber has a natural amount of expansion/shrinkage with humidity.
The larger the boards the larger the gap between boards.

Say a normal piece of wood shrinks/expands 10%.  I don't know the actual percentages.  This is just a round number for an easy example.  A 4 inch board will shrink 4/10 of an inch.  Leaving a .4 inch gap.  With a tongue and groove, not as noticeable a gap.  Double the size of the board, double the size of the gap.  As boards get wide, it can become noticeable.  There are stories of old houses that had 10" and 12" wide floor boards having to stuff rope or wool in between to keep out drafts.

That is why when you go to the store, flooring is in thinner strips.  It would be a lot less work and waste on the part of floor manufacturers and installers to not have to tongue and groove 2"-3" strips.

Since you are using a softwood instead of a hardwood, it shouldn't shrink as much.  Maybe someone that has better detail on actual shrinkage/expansion by species can chime in.

[Don_P]

Well, the Shrinkulator of course 
http://www.woodbin.com/calcs/shrinkulator.htm
You'll probably have mostly tangential grain with some quarter sawn in the mix. While looking at the numbers this should give one of the reasons why a quartersawn (radial grain) floor is considered the cat's meow.
The T&G joint keeps the pieces aligned but don't add strength, the 2x8 will be a stiffer floor. There is a greater likelihood of cupping, and bowed boards take more oomph to draw tight the bigger the piece. Roll thru the stacks and see if they are both the same quality and equally flat and straight. You would prefer very few hearts.

One of the biggest things with softwoods is getting them dry to begin with. Typically the moisture content is going to be either 19% or 15% if you're lucky. Hardwood flooring arrives at 8-10%. If you have time to sticker the lumber and let it acclimate for a few weeks it'll lose some of that moisture and that part of its' shrinkage. Then the movement is simply the fluctuation in indoor relative humidity seasonally.

In a fully climate controlled house there isn't much seasonal variation in rh and the wood doesn't move as much. Wide plank works well there. We've got the windows open and fans on now and will be running the stove all winter, all joints in our house are true working joints, the emc ranges close to 10%. Doors stick now and rattle in winter. I took up and reused a 150 year old random width wide plank poplar floor some years ago. The boards had gapped every winter, junk fell in the gaps and then was compressed as the wood swelled every summer. I could not clean the grooves out with a pressure washer,  had to make a wooden block plane with a groove blade to run down the grooves and clean the compacted trash out.

One other reason for narrow strip is yield. Narrow strip random length flooring can convert some really low grade logs into very nice flooring. One friend was selling the high grade logs he was harvesting to the sawmill and making flooring out of the low grade stuff.