If you compare Defy vs SE Xperia Mini Pro SK17i hardware wise, then most of the members here will suggest the SE Xperia mini pro.
Here is the comparison:
Sony Ericsson Xperia mini pro vs Motorola DEFY
Specs:
Motorola DEFY - Full phone specifications
Sony Ericsson Xperia mini pro - Full phone specifications
Tell us your main requirements. What do you expect from the phone?
Choose Xperia if:
You want faster processor for faster performance, better multitasking, better and faster gaming experience, Bravia LED backlit Lighting, Android 2.3 out of the box, 5 MP Camera with LED Flash and 720P video recording, Better music than Defy, Keypad.
Do not choose Xperia if:
You do browsing/watching movies and are uncomfortable in looking at the small screen, You want a phone to be water/dust or pretty much lifeproof.
Choose Defy if:
If you want phone that can withstand anything and everything that happens to it, Waterproof/dustproof phone, Decent processor with bigger screen (.5 inches bigger), Better XDA support, 5MP Camera and LED Flash
Do not choose Defy if:
Want better gaming experience with newer games (this is in comparison to xperia mini's gpu), better camera performance with 720p recording and also good multimedia experience. Of course 720p video recording can be achieved by modding it.