You will have both option as we are covering all the 80-85 zones.
I find Vash'irj slightly slower initial since the Map is massive and you're forced to swim around at 60% speed. While in Hyjal you can benefit with the speed of your 310% mount straight away.
it is also possible to begin at level 78 however you may need to grind a bit of XP in dungeons as most of the quest require level 80 or switch between the 2 zones and complete all the level 78 quests.
My choice for speeding through the guide would be Hyjal but Vash have a more epic feel to it
so the entrance quest for hyjal starts in SW too or can I park my toon at winterspring to enter hyjal right after release? heard that the entrance to hyjal is at that area but i may be wrong.
Mount Hyjal: You will be sent to talk to Druid in Stormwind City / Orgrimmar which will port you to Moonglade and from Moonglade you get Dragon Ride for a little bit of Cinematic.
Vashj'ir: You will be sent to Stormwind Harbor and a boat will arrive take you to Vashj'ir. For Horde you will be sent to a new harbor North East of Durotar to take you Vashj'ir.
There are also handy portals for both zone from Stormwind / Orgrimmar.
Also Flight Trainers for flying in Azeroth for Flight Master's License can be found in Stormwind and Orgrimmar flight path area. It will cost around 250g,
and 310% flying speed or Master Riding can be purchased for 5000g.. both can be discounted by 20% if you're exalted to the city faction I think so raise your reputation for some savings.
by bein able to fly around in the hyjal, wont i miss alot of xp by flying from questgiver to "quest spot" instead of walkin and killin mobs on my way to the spots?