Here is one meta analytic comparison:
Bernard M. Bass is one of the most cited researchers on this field:
Here is one of his articles but there are plenty on the net:
I think he is also author of MLQ - multifaceted survey of leadership.
There is also revised theory by Bass and Avolio:
The implications of transactional and transformational leadership.
And also: Re-examining the components of transformational and transactional leadership using the Multi-factor Leadership; Journal of Occupational and organizational psychology. 2010.