They are similar in nature, both being built from the same source (Official Sprint/LG VD) but nick tweaks some things we don't and we tweak some thing he doesn't. Does that make one better than the other? Nope, not IMHO. It means if the things he tweaks are more important to you and as a user, use nROM. Keep in mind if you end up liking nrom but want the theme of Thunder ... you can find the ThundeROM theme in the LEGO thread. Conversely, if you like ThundeROM but like the theme of nROM, you can find that theme in the LEGO ROM thread.
I've used multiple android devices and typically what happens is theres a "fight". A bunch of developers fight each other for who can do what better faster and better than the rest. That competition prevents information sharing between developers. In the Optimus S world, Nick, myself, and Beezy all have an established understanding that what we do is bigger than 3 guys, it's about the 1,000's of people who use the work. None of us have any qualms about sharing info to help the other. In fact, ask nick ... if we have a nice release he's the first to gtalk me and let me know, conversely, if he has one I'm gtalking him calling him names i cant say on here

(because i know he's pushing my limits too)
As an end user, you should be proud to own the Optimus S. If you like nROM, man i don't hate you at all, i dig the fact you're willing to explore. If you like ThundeROM, i love you!!! kidding!!! Bottom line is enjoy it because I'm not sure the same information sharing system will happen again anytime soon on another phone.