Hi,
If you're looking for a good banking job, I'm afraid you won't find one through any of those agencies unless perhaps, you go through a specialised agency such as Randstad (
www.randstad.es )
If you just want an internship, I suppose that going through one of the agencies would not be a bad idea....but if you want something specialised and related to your career, your best (and perhaps only) bet is to go there and then look for an internship. It would be easier obviously, to get an unpaid internship and I'm sure many companies could use the extra help