[IamPRO]

Yes there is a difference:
in the first code.

pawn Код:

public OnPlayerPickUpDynamicPickup(playerid, pickupid)
{
    if(pickupid == RandPick[0])
    {
        cmd_cnr(playerid,"");
        return 1;//returning to 1 ( Ending OnPlayerPickUpDynamicPickup)
    }
    //code here will not be executed.
    return 1;//returning to 1
}

in the second code

pawn Код:

public OnPlayerPickUpDynamicPickup(playerid, pickupid)
{
    if(pickupid == RandPick[0])
    {
        cmd_cnr(playerid,"");
    }
    //whole code will be read when callback is called
    return 1;//returning to 1
}