I was wrong since I considered in the calculus the 15% witholding that as a foreign investor I have to pay for preferred but not for senior notes/bonds (something I don't know the reason why but it's like that).
as of today:
WRT @25.59 current yield = 7.57%, YTM = 6.85%
FURpD @26.70 current yield = 8.66% YTM = 7.31%
for a US investor the preferred is still more convenient.