http://www.vegagerdin.is/english/road-conditions-and-weather/condition-of-mountain-tracks/
This is the background for opening and closing the various mountain roads. Unfortunatly it only contains the historical opening dates, as closing the roads are usually local weather dependent. Still that page gives good information.
F26 is the Sprengisandur route and not really suited for that time of year. I drove it 10th of august 2013. 3 weeks later there where sections of snow that required vehicles to convoy and first vehicle to have front and rear lockers activated. It is a remote, corrugated track and at that time of year a vehicle failure is very bad. During highseason you will see 3-4 other travelers per day. At the low season, possibly one every 4-5 days. There is no cellphone coverage that you can rely on.
F35 is the Kjölur route where I would hope to cross. We did that 15th september 2009 and it was perfectly fine. It's better maintained gravel road, more people travel there. There is more variation in the landscape.
There is no prediction on where it will rain more. The only general prediction that you could follow is that snow tends to come in the north and in the highlands first, the delay before it hit's the south coast could be as much as 2 weeks.
Nature is all around Iceland, so what will you miss by going in one direction, compared to another? With the only exception being the Geyser, that you will only find in Geysir area located towards the south - east corner. Other than that, there is plenty of fjords, valleys, waterfalls, mountains, breathtaking scenery, lush green and barren landscapes to keep everybody happy. With a generous amount of glaciers and volcanic deserts, you are never really far from adventure.
One thing I learned during 22.000km up there, is that when you think you have seen it all, you stumple upon something new.
Now we are waiting for Hekla to erupt before going up there again, I would love to see that awsome display of power.
http://www.youtube.com/watch?v=lrNfB8CMGtc
Here is a small video I made, when we went up there to store the car, waiting for the main trip during summer.
http://ourfootprints.de/gps/gps.html Very good GPS map of Iceland, if you got Garmin MapSource