Either one should be fine. There about the same core clock speed
and both hav the Phenom II micro architechture.
For lower pwr considerations the 705e (65w) would be the better choice but
at a higher price point, about a $30 difference.
The X3 720 is a black ed and only is generally $20 more than the 710. Its
95w like the 710 but if your lookin for better performance speed and ur not
goin to o'clock then why not get the 720 Black ed. (95w). It also has the
advantage of a 200 mhz faster core clock and for future perfprmance considerations I think it a better choice than the 710 for only $20 more.
The other consideration is your GPU. Even with a HTPC and X3 I would still
want a GPU that will 100% off-load the CPU. I hav a Phenom X3 8650
presently married to a onboard RadeonHD3200 from the Biostar TA 780G uAtx
MB. Blue Ray HD does fine
Hope this helpd.