K-Tai Watch had a handy chart comparing the costs of new FOMA "All You Can PACKET!" unlimited data plan with au's "EZ Flat" unlimited data plan.
And I ripped it off.

NTT DoCoMo will begin offering the unlimited data service, "All You Can PACKET!" o­n June 1st. The price for usage of this will be 3,900 yen per month. au CDMA 1X WIN has offered a similar service since last year, called "EZ Flat" - for the price of 4,200 yen per month. Let's compare the standard usage costs, and free talk time.

When comparing just NTT DoCoMo's "All You Can PACKET!" and "EZ Flat," we see that NTT is 300 yen cheaper. However, since the usage of both of these services requires a minimum service plan, when we put in the minimum, we see that au is in fact cheaper. This is a summary of the prices and minutes of the two plans.

Simple Plan Comparison

-NTT DoCoMo FOMA
Fee Plan	Monthly Basic Charge	All You Can PACKET!	Total	Free Talk Time
Plan 67	6,700 yen	3,900 yen	10,600 yen	155.8 minutes
Plan 100	10,000 yen		13,900 yen	306.3 minutes
Plan 150	15,000 yen		18,900 yen	529.5 minutes

-au CDMA 1X WIN
Fee Plan	Monthly Basic Charge	EZ Flat	Total	Free Talk Time
Plan SS	3,900 yen	4,200 yen	8,100 yen	25 minutes
Plan S	4,900 yen		9,100 yen	65.6 minutes
Plan M	6,900 yen		11,100 yen	150 minutes
Plan L	10,000 yen		14,200 yen	275 minutes

Clearly, the cheapest is "Plan SS" at 8,100 yen. In terms of free minutes, however, NTT DoCoMo offers many more for the 2,500 yen difference.

Inspired by:
http://k-tai.impress.co.jp/cda/article/news_toppage/18227.html