Hi Brian and WELCOME ABOARD...!
Given the two choices, I suspect the quality is not that all that different which leads me to suggest going with your personal preference.
Mamod is a stalwart of the UK and the remaining manufacturer in the UK.
Wilesco, while a newer player in the steam toys (1950's) has more of the realistic details typically associated with German engines.
Both will bring you joy for many years to come.
Gil