I like the large version now
PNG should be allright. Most browsers support them 100% (more if they dont use alpha channels.
(the only thing are those animated pngs. Havent seen lots of support from those since the support withdrew on that initative.)
I would say its all in the generated file size comparison. Might even think about a .gif comparison since it's about the same color on the same lines.
PLay around with the settings and see what the best quality between size and quality you get.