Little buttons to take you backwards and forwards might be quite messy. It's tricky to put them into the tweets (i.e. I haven't worked out how to do it yet - it may need some tricky jQuery).
The default approach puts them on an extra line - which I'd rather not do.
As for transition options, try http://downloads.wordpress.org/plugin/rotatingtweets.zip
I've limited the options for the widgets to try and keep things manageable, but if you use the shortcode, you can try everything available at http://jquery.malsup.com/cycle/browser.html via something like
[rotatingtweets screen_name='mpntod' rotation_type='toss']
Let me know if there's an effect you'd particularly like me to add to the widget! :-)