Plasmas work better in brightly lit rooms because they are brighter and have more contrast. Their downside is that the glass may create unwanted reflections, so you may need curtains to control this. LCDs have tighter pixels and generally do well displaying higher resolutions, but since you're viewing from 6 to 8 feet, this should not be an issue. Plasmas are better in the bad pixels category and have better response time, but they are worse in power consumption. Prices are somewhat close, but some of the deals I see in the 42 inch size for plasma are:
http://www.amazon.com/gp/product/B00142HLV4/102-3379922-5038537?ie=UTF8&tag=everyprice-20&linkCode=xm2&camp=1789&creativeASIN=B00142HLV4 or
http://www.techbargains.com/news_displayItem.cfm/144430.
I don't think the 720p resolution is that much of a factor at that distance, but if it is, you might want to consider LCD:
http://www.samsclub.com/shopping/navigate.do?dest=5&item=414940
