Sorry to butt into this conversation but using "Newer DS" as a base for your mod is not bad at all, I mean it does have its disadvantages but if you think it would be better to work with "Newer DS" than with "NSMB" that's fine but if you think the opposite then both "Newer DS" and "NSMB" have their advantages and disadvantages here are some.
Advantages of using Newer DS as a base.
1- it has some ASM codes that were not used much by Newer Team and you can take full advantage of them.
2- better stability and performance since it has a little more internal RAM.
3- some improvements with respect to "NSMB"
4- you can add more ASM codes.
5- less development time.
Disadvantages of using Newer DS as a base.
1- some levels are complicated to edit since they use areas from other levels.
2-some sprite functions crash the game for example the buzzy beetle "Upside-down shell"
3-some sprites have complicated functions.
4-some tilesets have animations but their animation files are well hidden for example "Starry Space"
5-same as disadvantage 4 the BG's have well hidden files for example "Space Fall"
6-level maps are complicated to change (unless you have the knowledge to change them)
7-music is difficult to change since it has a soundfont set for it to work correctly
Now the advantages and disadvantages of using NSMB.
Advantages of using NSMB as a base.
1-you have a blank canvas to work on a mod from scratch
2-you can install ASM codes either with the traditional engine or with NCPatcher
3-you can change level maps easier
4-levels are easier to edit.
5-All tilesets and backgrounds are easy to modify or change.
6-All sprites work correctly.
7-Music is easy to modify or change.
Disadvantages of using NSMB as a base.
1-You have to install ASM codes from scratch and if you don't like the ASM code and you have a great advance in your mod, then there is no way to start over in a new ROM. (of course unless you make backup copies of your mod)
2-If you put a lot of sprites the game slows down. (Compared to Newer DS which can support more sprites I think XD)
3-Some graphics are complicated to edit like the logo and world icons. (Unless you use ASM codes)
4-Add files that require ASM codes. (Unless you have knowledge in that)
5-Modify the "Overlay" files because if you make a mistake the game can crash. (unless you have knowledge in that)
6-more development time.
well it's just my humble opinion don't take it seriously you are free to use what you feel more comfortable and by the way me and my friend "Ultimate MrL36" are working to improve the level editor for Newer DS created by "Helios" to make it more comfortable to edit Newer DS just that it is not finished yet we will publish it soon.