SEO Keyword Dilemma
 

We are looking at SEO as one of several ways of promoting our website but its really difficult to distinguish the good from the bad and the downright ugly in terms of agencies to use. For example we are proposing to use an SEO agency (who have good reviews) who have come up with a long list of interesting keywords. The problem is that the degree of competitiveness which is listed alongside each keyword varies dramatically from the results I achieved by keying in the same keywords into the seomoz (now moz.com) keyword tool. The agency says their results are achieved by using Google tools. Now feel entirely stuck and dont know what to do. I can understand some inherent difference between various tools but would expect a reasonable degree of consistency. Is it reasonable to rely on Google tools when at this stage we are looking at organic search only rather than Adwords?