As with any marketing activity, tradeshows are just one of many things you need to do. They are just another building brick.
As for tracking ROI - I love metrics. But you also need to understand that not everything you do has a directly measurable ROI. Sure, Adwords do. A salesperson does. The ROI for a tradeshow is a little murkier sometime. But for the most part, after attending you can get a good feel for whether it was worth it or not.
Finally - on who should attend. Well, it depends. Who usually attends for the other companies in there? Having your CEO attend can give you credibility if he has especial knowledge in the industry, is great with customers and likely buyers are high-ranking in their own organizations.. On the other side, having your CEO there can give the impression that you're too small.
Best of luck!