So if you decide to follow the Schlieffenplan and go west first then...

Pros:
-You get early cash from eliminating Belgium and Luxembourg
-You pressure the Entente powers so much, that they do often not invest much in technology, thus providing you with a possible lead here
-Depending on how far you go you will constantly decrease french morale by a huge amount aswell as granting Germany a huge amount of morale from occupyied territory in northern and eastern France
-Britian will be forced to send most of there troops towards France early on, otherwise you will defeat the French one on one
-As both the French and the Brits tend to scale the hardest out of all Entente powers in terms of MPP production and technology, you may provide yourself with an easier late game by hibiting there potential early on

Cons:
-Russia can and will likely force Austria-Hungary in a very precarious situation
-You will likely not take out Serbia until 1916
-Italy will also drain even more of Austria-Hungaries sparse ressources and troops
-If you dont manage to gain a significant foothold into France, you will likely get simply overrun by the pure amount of troops the Entente pumps out in early 1915
-Russia will get the opportunity to invest a lot in tech, which will make them a far more formidable opponent in the late war stages
-The UK doesnt need to provide the russians with supply convoys

If you do go east first and fortify in the west...

Pros:
-You will be able to wipe out Russia no matter what very early on
-You will get huge amounts of morale by occupation of vast amounts of land
-You will be able to wipe out Serbia really quick
-Austria-Hungary is in a far better position early on
-Once you have defeated Russia, you are free to use all CP forces against the western fronts respectively
-Britian will join a bit later

Cons:
-The French and Brits will scale up huge especially in technology its likely they will surpace you
-The British blockade will likely strangel german morale much harder then if you can compensate that by having Belgium and northern France occupied
-The British will likely rush down the ottomans quite easily
-French troops may likely be send to the Balkans and Italy which will improve the Entente strenght there significantly
-Once you defeated Russia the Bolsheviks influence (no matter if you actually accept the Brest-Litovsk treaty or deny it) will f..k your morale as CP very hard, grain supply from Ukraine wont compensate this at all
-The USA will likely join the war much earlier which is very dangerous

Going East first is the best option for winning Singelplayer matches, the AI isnt as aggressive in France and doesnt utilize there huge power very effectively later on. Your morale will usually also always stay around 100% because the AI doesnt focus on eliminating weak troops as often aswell as playing less aggressive then a human at all.

Going West is from my experience far better in Multiplayer matches, as you can deny early tech development and the human players will always be much better at impacting your countries morale,
especially after Russia has fallen the bolshevik influence combined with the naval blockade of Germany will destroy the CPs morale by at least 1-5% each turn.