Which phone is better my-touch 3G or iphone 3G [S]

Data Plan with T-Mobile USA - 24.99 unlimited data, 34.99 unlimited data and text
Data Plan with AT&T for iPhone - $30.00 unlimited data, $50.00 unlimited data and text.

MyTouch 3G T-Mobile:

Plus: Ability to change entire UI, including screen, keyboard; all aspects of phone can be user controlled due to open source interface.
Minus: Limited 3G / voice coverage due to late launch of 3G by T-Mobile.

iPhone 3GS:

Plus: Larger library of applications, wide assortment of accessories and docks.
Minus: Limited interface changes (needs to be jail broken for any real UI changes), limited options for video and audio streaming. Frequent software updates often create new problems while resolving others.

I'd rather you draw your own conclusions based on the information above. Both phones do roughly the same things, especially now that the iPhone supports MMS.

I see longer term growth in Android, as many major manufacturers are coming out with phones in time for the holiday season (Motorola Cliq launching soon on T-Mobile, HTC Hero launched on Sprint October 2009), which may see Android outstripping the iPhone for units sold.

Personally, I've owned both phones, and prefer the customization options and keyboard on the HTC Android. However, based on network in your area (check coverage maps first) and who you call most might be another factor to consider before deciding which one is better.