I am not 100% sure on all the differences between the 606 and 705. From what I understand the 606 is the replacement upgrade for the 605 with 2 extra HDMI input for a total of 4 (the 705 has 3). The 705 has better video processing with 3 chips vs 1, a bit more power (10w per chn.) and is THX certified (if that matteres to you). You can look at the comparison between the 2 at and see all the differences at:
http://www.us.onkyo.com
Just go to products/receivers on the top, check the 606 and 705 and hit compare for a side by side....
If the extra $100 is a bit too much, the 606 will be fine and the extra HDMI can be a bonus. But the 705 is worth the extra $$ IMO..