Your suggested use of AdGroups sounds good to me.  I would use a different AdGroup for each landing page or each theme, as use suggest.  [Google's AdGroup documentation says](https://support.google.com/adwords/answer/6298?hl=en):

> We recommend that you create a separate ad group for each theme such as for each product you offer (like wedding catering and party catering), selling points (like free consultation and gourmet menus), or ways to describe your business (like caterer and on-site food service). The ads and keywords in each ad group should directly relate to that group's theme.

As far as measuring goes, assign a dollar value to everything: 

 - Track concrete sales and assign the profit dollar value from that sale (ignoring the cost of AdWords)
 - Figure out the value of a lead acquisition.  Assign AdWords the value that you would expect to pay for an acquisition from other channels.

With these figures in hand, compare how much you are spending, to how much value you are getting.  Adjust your bids such that you are not bidding above this value.  In my experience, you do best when you bit at 70% to 90% of what you expect take make.  So for example, if you can make $0.50 per visitor from AdWords, then a bid of $0.38 would be reasonable.

I tend not to assign much value to increased traffic itself.  If two bids will yield the same traffic but one has higher volume, I will tend to use the higher volume as the tie breaker.