I would deffinately say go for the yamaha you have listed over the others. Alot of people on the internet seem to like that Oppo player so i guess its one to consider as well.
If your Sony 34" has no HDMI connection and you are buying the DVD player for viewing DVDs then I would go with the 971HD.
However, if you have a HDMI connection, and seeing that your TV is less than 50" then the 970 would be a good choice because of the HDMI.
I have the Oppo 970 now connected to a 40" Samsung LCD and it looks spectacular connected via HDMI. I have never had a 971 but the lack of HDMI output kept me away from that. I hear Oppo will be coming out with a new version of the 971 with HDMI.
John: Suggestion, download both user manuals for the 970 and 971 from the Oppo website. Also notice there is a setup guide published for the 970 as well. The Oppo FAQ is also very informative.
Arty G The TV does not have an HDMI connection but a DVI. I was thinking of going with the 970 since my screen is less then 50 inches. but I will need to buy a hmdi to dvi adapter