Up1 looks nice, but is internally using SJCL which is slow and does not support bigger files ("This is not a problem with sub-10MB images"). Send - and Whisply - are built on the new WebCrypto APIs which are faster and allow bigger files, up to 1 GB in both cases.
PS: Whisply even has more features and a detailed description how its encryption works: https://whisp.ly/static/whisplyTechnicalOverview_20151201.pd...