Freeset is the better long term bet in terms of future HD channels, simply because of the limited space on Freeview at present. However, that doesn't automatically mean Freesat will get any new HD channels before Freeview, or even instead of Freeview. In fact, Channel 5 HD is expected to launch on Freeview around March/April next year whereas they have not mentioned Freesat in their blurp at all.
If you have a satellite dish already, then Freesat is a good option. More channels than Freeview and longer term, more capacity. But it doesn't have some of the Freeview channels (Dave, Five*, Five USA etc.) as these are still encrypted on Sky.
Freeview is probably the simplest to go with if you don't have a dish already. Decent but not spectacular range of channels, good range of equipment available and, in terms of SD only, by far the cheapest route to digital TV.
Which one you should go for depends on what channels you feel you are likely to watch most often.
When it comes to picture quality, well that's a subjective issue. There isn't any difference I feel between the HD channels on Freesat and on Freeview. On SD some channels are better on Freesat, while some appear better on Freeview though.
