That really depends, the M&P course (at least in year one) covers some extra Maths things (I think Calculus and Algebra) whilst the TP course has labs (both intro experimental stuff and their computer labs) and covers the Theoretical Physics module (Mathematical Modelling). TP also has the Physics mathematics course which covers mathematical things that are needed for the other Physics modules and a Python intro (which I don't think M&P gets). So generally I would say that if you prefer Physics or computer stuff, go for TP, or if you prefer Maths, go for the M&P course.
The theoretical physics programme allow students to explore the physical world through computational knowledge, creating simulations of complex systems through computational laboratories. Theoretical students explore the mathematical structure of physics and build mathematical and computational tools to explore physics phenomena.
Maths and Physics students studying the joint honours programme investigate the practical application of numbers by studying the rich structure between both subjects. Students explore the challenge of proving the laws of nature from first principles and are taught by experts within each field. Approximately half of the programme will be spent in each department.
Both courses are accredited by the Institute of Physics and cover a common core of modules giving students a thorough grounding in physics knowledge and scientific principles. However, a full list of the modules covered in each programme can be found by visiting: http://www.york.ac.uk/physics/undergraduate/modules/.