I would probably suggest the MX5021s, since they are newer and come with dual midrange drivers. I'm not sure if Altec Lansing is a true 2.5-way but given even Logitech is crossing over its dual-midrange speakers I imagine they would to avoid combing.
The CS21s look good but performance wise, but the I would take the MX5021s for the piano black finish even if they were equally performing speakers.