They both have charm. For TEs, Wilesco is more realistic but Mamod TEs allow Mamodification. Mamod steam wagon lorry and TE is great fun to drive with the steering rod. For stationary engines, Wilesco has a good amount of grunt.
Honestly, the quality of the later Mamods are better than the earlier ones. (Sight glass, better cylinder and piston designs, etc.)
I strongly dislike Esbit (gives headache and sore throat) so the Wilesco burners are annoying. I remove them to allow spirit or gel burners. Older Mamod spirit burners are fantastic. The great news is that all Mamod engines work with their spirit burners.