UI design when Developing 1 mobile app for both iPhone and android

As a developer What are your thoughts on UI design when Developing 1 mobile app for both iPhone and android..

Since the application will not be developed for a specific mobile device there are two options in regards to UI

1. build the application using native UI (one for iphone, another for android)

2. build the application using custom UI (same custom UI graphics used for both开发者_开发问答 iphone and android). (like paypal?)

Which option is best and why? (saves time, looks and feels better? Etc)

Use the native UI framework for each app, if you can. You'll have two apps that look good for each platform, instead of one app that looks a little out of place on each platform.