We need to buy a headset and microphone which will be used for doing a webinar and training an audience of 140 people who will be in different locations(mostly in same city). The speaker/presenter is in his mid sixties.

Any specific headset with a microphone should I look into which has particular features or any headset microphone
like http://h10010.www1.hp.com/wwpc/ca/en/sm/WF06c/A1-329290-64273-356878-356878-448954-3737178.html

or the ones at

http://accessories.us.dell.com/sna/productdetail.aspx?c=us&l=en&sku=A1282206

would do which are priced for around 20 to 49? Would the 20$ do for what we need or should we choose the 49$?

I have not done such a training webinar before so any advice would be welcome.