Really, it depends on what you look for in a fighter...
ROTD is a not-as-serious fighter with VERY good graphics and lots of color. The system is decent, if not simple. Simple doesn't necessarily mean easy, though. It's also very "off-pace" which means if you've spent your life working on your timing in fighting games, it'll take you a little while to get used to ROTD's timing. I still don't have it down and get my ass handed to me *Still* in ROTD.
NM is a more serious fighter with a different but no deeper or more involved fighting system. The graphics are not as colorful as ROTD and they're more mid-90's fusion, but that doesn't mean they suck. NM is all about mood and atmosphere. It's also love-or-hate. There's little middle ground.
They both have a dial-a-combo system with ROTD having the element of Tag Team and Duplex moves while NM has Armed and hand-to-hand fighting. It's a push for me when it comes to these games in this respect with ROTD getting the nod on some days when I want a fighter to be fast-paced and over-the-top and NM getting the nod on rainy days when I want to learn some new strategy or really get into the game for a few hours and push myself to be a better player.
It's up to you, I hope that helps.