The obvious comparison is between iphone and android, but with each side, especially android, gaining an increasing number of models it is no longer a simple comparison.
On the iphone side there are 4 phones, the original iphone, iphone 3G, iphone 3GS and the most recent iphone 4. The difference between the first 3 iphones specs is negligible, infact the original iphones processor was underclocked (set to run slower) and by jailbreaking it was possible to revert it at the expense of battery life. The iphone 4 is substantially more powerful than previous models sporting a 1 ghz processor.
Androids phones have models on every market level. The gaming capability of android phones is directly related to their price point and even phones designed specifically with gaming in mind, such as the Sony Ericsson Xperia Play, aren't necessarily more capable of running games. Because of the diversity of android phones there are many high end phones with remarkable specs. The recently released Galaxys S 2 utilizes a dual core 1.2 ghz processor making it far more powerful than the iphone 4.
The gaming capabilities of a phone however, are not determined only by their power. The standardization of iphones allow developers to optimize the game for that device, because of the large number of android devices it is no cost effective for a developer to optimize the game for only one android phone.
Deciding between standardization and customization is a tough choice, and can only be made by the user. Over time we may start to see developers offer android device specific optimized games, as some phones rise to popularity, but for now it is a case of power vs precision.