Hilton is located in Puerto Madero, an area full of bars and restaurants that you can walk to at night as it feels very safe. It´s a much newer hotel with a modern feeling to it.

Marriot is located in Calle Florida in Plaza San Martin. This is a really turistic area during the day and can feel not very safe at night. Be very carefull with your belongings in Calle Florida during the day. Marriot is quite a "classic" and old style hotel.

I would go for the Hilton.

Taxis are plentiful and cheap in BA, so you´ll be able to move around freely.