The simple answer is if you want more choice of HD content then get Sky even tough I have read that a lot of the Sky HD is actually upscaled SD, I am not sure if that is true. Obviousely you have to pay and as said if you quite your sub after the year you can not record unless you pay Sky £10 a month.
With the Humax you only get BBC HD, ITV HD via the red button and they do not show too much in HD as yet. You also get Luxe HD in non Freesat mode. It is anticipated that CH4 HD will come to Freesat this year and I would assume and hope that in time there will be more HD content on Freesat.
As said by someone else the Humax at some point will support the iPlayer and also the Humax upscales SD, not sure if the the Sky boxes do.
And of course with the Humax box you can record what you want when you want until your heart is content without having to pay anyone anything!!
So if your worried about lots of HD content you have no option but to go for the Sky box and contract at the moment but if you are not then I would go for the Humax Foxsat HDR any day, as I did.
One last thing, a friend of mine who has Sky+HD says that the pq on SD and BBC HD is better on mine then his. That could be my TV, the way I have it calibrated or it could be down to the Humax.