^Yeah, stupid war on drugs

Harm reduction supplies should be readily available. At least you sound knowledgeable about what you are doing and the risks you are taking and are looking for ways to reduce them
Sorry that I don't really know whether there is a dramatic difference between prepping or safety between the two brands either. Hopefully someone will post who has experience with IVing the Mylan ones.
Here's what I know about some of the ingredients in case it helps:
The ingredients the Mylans contain that are not contained in the MS Contins are:
- the pigments: I have no idea about these but
assume it shouldn't make too much difference (?)
- colloidal silicon dioxide: this sounds like a
bad idea to inject, insoluble in water, can pass through a cotton or cig filter (and maybe even a micron filter? although the units tend to fuse together in chains so I'm not sure what the actual particle size is), made from silica which is what's in sand and glass

- lactose monohydrate: basically milk sugar + water, a lot of street drugs are cut with lactose so I would
assume it is not
too bad (?), it's listed as "partially soluble in cold water, hot water"
- polyvinyl alcohol: a water-soluble synthetic polymer, used in eye drops but I don't know about injection
- presumably the quantities of inactives as well
Found in both brands:
- hypromellose: can gel up when you add water and especially when you add heat
- talc: not safe to inject - Google "injecting talc"
- magnesium stearate: not soluble in water, found in almost all pills unfortunately
Some related reading (unfortunately it's a study about prepping the MS Contins for injection but it may be of interest to you)