Pre-MBA is a kind of foundation course which prepares you to get entry to MBA of that specific uni,which is offering PRE-MBA.
Visa chances for direct MBA is far better than pre-MBA.
As far as entering into direct MBA they require GMAT..... which is a Tough Nut to Crack atleast for me..... for that reason i am going for Pre-MBA...... dun know about the Visa that whether i would get it on Pre-MBA or not....