I used to be a trader at DB London in Fixed Income, i.e. bonds, Financing and Lending (known as the Repo Desk) and also traded Forward FX at Mizuho. It may seem a little biased if I suggest you choose these two desks but I'll try and justify the choice rationally.
By choosing forward FX you not only gain a knowledge of Spot FX but also a knowledge of cash deposits and interest rates futures due to the interdependent nature of these markets.
By choosing Fixed Income Financing and Lending, i.e. Repo, you again get an insight into the cash deposit market as you are lending out cash against the bonds as collateral on the cash loan. You will also gain an insight into Fixed Income markets and Fixed Income Futures. The Repo desk is a very profitable desk within DB London Fixed Income and well respected within the wider Repo market.
Having a solid first hand trading knowledge of the interrelationships between global markets will stand you in good stead for the future if you wish to climb the food chain beyond Desk Chief Trader level.
BTW, most institutional traders wouldn't survive long outside the corporate environment and when it comes to raw (proprietary) trading skill are a fairly overrated bunch. When you're there, ask some of the more seasoned traders the question: "Why are there so few dealers over 40?" The answers, if they are being honest, will give you an insight into institutional trading as a career choice and may help you decide what is right for you.
Best of luck and enjoy the experience
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