Should I save up for Suleiman or buy Nobunaga?

Buying Suleiman at this stage of the game is not a blunder, but if you want to hire a cavalry general then Attila is more useful and easy-to-use.
Nobunaga is, of course, one of the most powerful generals in the game and a perfect candidate for many team compositions. Yet, if your previous experience with EW5 is limited I would say Attila is your man. His active Commander skill and ability to attack without retaliation is an excellent choice for those who haven't played EW5 much or had troubles previously.