The second one (personal openion)! That seems a bit darker but more close to the Ubuntu orange (as specified in Ubuntu's Brand Assets). While the other one is far from this (but seems that's what recommended by Jono Bacon, Ubuntu Community Manager, in his website as linked by Oli in his post) and looks somewhat a blend of the community-feel Ubuntu Orange and commercial-indicating Canonical Aubergine, and may be that's the intention - cover both community and commercial software packages with that shade ?!
1: Ubuntu Orange - Canonical Aubergine blend:
2: The colour palette:
snap2 http://design.ubuntu.com/wp-content/uploads/ubuntu-colour-tints.png
Button1 pic1 http://hostmar.co/software-banner
Button2
Some guidelines for correct use of Ubuntu logo; you must not…
Change the spacing between the individual elements or characters
Change the colour of the wordmark
Change the colour of the Circle of Friends
Change the arrangement of the elements
Place the logo at an angle other than horizontal or vertical
Change the proportion of the logo
Place the logo on a non brand colour
Some easily noticed features in the compared buttons:
Ubuntu Orange? Or Orange? [Button1 - ???; Button2 + orange blends]
Colour of Circle of Friends? [Button1 - ???; Button2 + white]
Roundel? It's colour? [Button1 + white; Button2 - ???]
Looks good? [Button1 + Y; Button2 - ?]
Something probably needs an update..