More or less what has been said till now.

I've only been in the subway once and foiled a pickpocket...I much prefer taxis but wouldn't let that one incident put me off the subway. Don't worry about your lack of Spanish....Argentines are patient people and will tolerate your bumblings....they have to do that with me every day.
Despite what you may have heard or read Buenos Aires is a safe city for the tourist as long as you follow the rules.....be wise and you'll be safe.

Enjoy your holiday. ^