Returns may be practically same when compared both.
But interest rate are fixed and dividends from preference shares vary.
At times shareholders can earn more dividends.

Shares are assets of company and debentures are liabilities so.

you see, shares gives the right to the shareholders to become the owner of the company, while debentures are usualy loan taken from the debentureholders and are to be redeemed within a stipulated time, while now a days u cant even issue irredeemable pref shares. But yet per my own opinion you should invest in pref shares