There are several things going on here when it comes to NS-10M's
As butterboxes said, "that's where I learned to mix on in the beginning, and that's how I 'like' my monitors to sound like".
I'm sure that if someone has had the NS-10M's long enough, hell, ANY monitor long enough, they know them so well that they could PROBABLY use them as their sole monitors.
brianaustinny never said if he wanted either the HS50's or NS-10M's as his main and ONLY monitors, or as a secondary set of monitors to cross check his mixes on.
He also never said what his needs were.
Is he a songwriter with aspirations of being one who can make a living doing so, is he a hobbyist who just wants to do this for the pure enjoyment of the process, is he someone who wants to be an engineer, someone who wants to master recordings?
As I said, there are several things going on here when someone makes a decision that they want to invest in recording gear.
IMHO, if one has never owned the NS-10M's and can only buy one set of monitors, I would go a totally different route, as there are many choices available nowadays in the price range stated.
Not to mention that one needs a really good amp (read, expensive

) to truly take advantage of them.
That being said.........
if brianaustinny wants a second set of monitors and is willing to spend around $650.00 to $800.00 for a set of NS-10M Studios (the most desirable version), and another $1000.00 plus on a great amp, why not buy them?
that being said.........
I might go the Auratone 5C route and spend the savings on other improvements for my studio