I loved San Angel Inn, I am not sure why it gets a bad rap, except a lot of people go expecting texmex not traditional mexican.
The food we had there in march was great, not to mention the staff was courteous and very accomidating. (dh drinks a lot).
And the atmosphere, well so far I haven't found anything to beat it yet. It is fancy enough to feel so yet you don't feel out of place being in "park" clothes. I would eat there again in a heartbeat.
I have not eaten at the other place you mention, nor would I. Give San Angel a shot, seriously I don't think it is a mistake.