Definitely go via AEP. Buenos Aires is cheaper, more interesting and you would want to come back for more. I can squeeze Recolecta, Puerto Madero, San Telmo and Boca all in one way at a leisurely peace and have great steak lunch and/or dinner, no worries.

Not sure about what exactly to see in Palermo - it's a (comparatively) upscale residential neiborhood alright, with fashion stores, cafes, nightclubs etc. I feel it's like Ipanema in Rio de Janeiro without the beach . Good place to stay if one is to stay in BsAs for a while though.