I've been doing exactly what you're contemplating doing -- having both
TeX Live and
MiKTeX on the same system and being able to access one or the other distribution -- for about two years. My front-end is the program editor "winedt" (version 5.6), and all I have to do to switch from one system to the other is to go to "Options" -> "Configurations" and choose between "MiKTeX (default)" and "TeX Live"; winedt will then reset all internal path variables. In addition, I've also installed TeXworks on this system and configured it to run TeXlive exclusively, as I couldn't be bothered to figure out how to switch all the path variables.
In my experience -- related here with absolutely no claim as to being representative -- MiKTeX may be a bit slower than TeX Live, but it also has better "crash recovery" in the sense that if I'm running MiKTeX, it's not as frequently necessary to blow away all aux files if the compilation ended with a crash somewhere before being able to recompile the LaTeX program. Oh, and when switching from MiKTeX to TeX Live, I usually (though not always, depending mainly on how many packages needed to be loaded) have to delete all aux files before I can (re)compile the file under TeX Live. Vice versa, when switching from TeX Live to MiKTeX, I've yet to encounter this particular phenomenon (viz., having to delete all aux files).
The main downside of having two full TeX distributions is that you must keep track of two separate upgrade and update systems. In my experience (again: no claim to being representative), new and updated packages tend to become available on TeXlive usually a few days sooner than they are on MiKTeX. This experience may be mostly a function of the mirror sites I use to access the respective distributions.