With this particular keyword phrase, I'm not surprised at the difference between the two search estimates.
If broad match targeting is selected for this phrase, ads would be eligible to show for searches containing the words find and deal, either together or separate, and possibly with other words included in the search query.
If phrase match targeting is selected, ads would be eligible to show for searches including the words 'find a deal' in that order (I don't believe the "a" is factored in), in addition to searches including other words, as long as the keyword phrase is in order within the query.
See: http://adwords.google.com/support/aw/bin/answer.py?hl=en&answer=6100 for more details on keyword match types.
Without doing any additional research, I would suspect that the keyword phrase "Find a Deal" when targeted with broad match would include wide ranging possible searches, e.g. "Vacation Deal", "Deal on Umbrellas", "Find Shoe Stores", etc. Obviously, the numbers from broad match targeting here are going to be higher in volume but much lower in relevancy. In contrast, analyzing "Find a Deal" with phrase match is going to be limited to searches such as "find a deal on electronics", "how to find a deal on hotels", "find a deal on Seattle hotels", etc. This limitation should result in lower impressions but greater relevancy and conversion rates.
So as far as what match type numbers are most important to you, I would say that it depends on what your business offers and how precise you have to be with your targeting in order to attract a relevant audience. If you offer every possible deal on anything under the sun, broad match becomes a bit more attractive. However, I imagine this isn't the reality of the situation, in which case you'll end up often having a mismatch between user demand and website supply, as well as lower CTR and CPA with higher cost if employing paid promotion.