Which is superior, CSS transparency or PNG Transparency?

Question

Should I use PNG transparency or CSS transparency?

My first instinct is to go with CSS. However I heard somewhere that the filter in IE is slow and that I would get

Answer 1

I just did a quick non-scientific test in Firefox 3.0.11 on the mac.

My test was to overlay a div and scroll up and down repeatedly.

One used css:

background: #000; opacity:0.8;

The other used a 10px 24bit PNG of the same.

I measured the following results with Activity monitor (so guessing,not exact)

Firefox 3.0.11

CSS opacity: approximately 60% cpu usage.

PNG: approximately 20% cpu usage.

Safari 4.0:

CSS opacity: approximately 80% (with peaks of 120%!)

PNG: approximately 76% (with no peaks at all)

So as far as performance goes, it seems like a PNG is the winner.