If you have NS-10s then stick with them.
Two reasons-
#1 - Industry standard.
#2 - Last I heard, they stopped makin' them - something about the scarceity of the trees the cones were derived from...

Also, I believe the HS50M is the "replacement model" for the NS-10.
I could be wrong, but I seem to remember reading that somewhere as well.

There is nothing honest about Ns10s they just sound bad and maybe thats just what you need to make your mix sound right?

The best thing to do is to go and give a few different makes and models a listen.

Everyone's ears are different, so are everyone's cabinets.

If you can, take a favorite CD with you and listen to it on as many cab's as you can.

Try making a mix of a variety of music - bass heavy, classic rock, new rock, and country - and see which monitors sound most pleasing (and honest) to you!

Read some reviews first.
See if you can find other people that use them.

I don't want to go on record telling anyone to jump into some new monitors without hearing them first, because like I said earlier, everyone's ears are different.

I get some great mixes out of my EV S-60s, but that doesn't mean that they're a good fit for somebody else...

The HS50M has absolutely zero connection with the NS10s. Only things in common is the name of the maker and the colour of bass cone (white). Nothing at all else in common. They are definitely not any replacement for the NS10, I would be extremely surprised to find them in a large studio.

Instead they are one of the many smaller monitors with built-in amplifiers. I guess you can find at least 50 different types for sale right now. All with their pluses and minuses. As such the HS50M are not bad, but not really stellar either.

Gunnar

OK, not a replacement. But depending on who you talk to they do appear to come off as similar. At any rate, here's what a few different folks have to say a bout them.

From Yamaha.com:

The new HS Series powered monitors were designed to be true studio reference monitors in the tradition of the famous NS10Ms. When choosing your next pair of near field reference monitors, remember it's not just important that they sound good; they have to be an honest reference for your mix. The new HS Series of reference monitors were designed to give you exceptionally flat response so you can really trust what you hear. That means that mixes that sound good on Yamaha HS speakers will sound good on anything. In fact, that's the ultimate test of a reference monitor. Even better than that, HS series speakers not only sound good, they look great, too.

From Mix Magazine:

The HS50M would be my first choice (among current production models) for a band-limited mix reference mounted atop a console. Similar to the NS-10M, the HS50M makes it easy to balance levels for a mix's midrange elements. Unlike the NS-10M, the HS50M readily reveals vocal sibilance and cymbals that are EQ'd too crisply. The HS50M provides a nearly perfect reference for how a mix will sound on small consumer-playback systems and is a veritable bargain at $249.95 MSRP (sold singly). Add the perfectly matched HS10W ($599 MSRP) to the setup, and you have an excellent full-bandwidth system.

To be fair, this is from EQ:

There’s been a lot of talk about these monitors, especially the HS 50Ms. They’re close in size to Yamaha’s famous NS10Ms, and look similar as they also have a white woofer. So are they the same, and do they sound as good?

Let me start off with saying it’s not possible for the HS 50s to sound like the NS10Ms. For starters, NS10Ms have a sealed baffle design while the HS 50Ms are rear ported. Sealed boxes have a different distortion profile compared to a vented box. The new HS series also have different cone material and crossovers. Every cone material has its own signature so you’ll get a different sound with each material. NS10Ms have fast impulse response and gentle phase shifts with the low frequencies. Though this design can be helpful, generally the bass response in a sealed cabinet is limited.