The only problem with No$GBA is that it doesn't allow you to go underground in the Pokemon games. I have Platinum as well, and have noticed the Desmume slowdown. No$GBA is definitely a faster emulator for Pokemon, especially when using No$Zoomer with it.
As a result, I only use Desmume for going underground--in fact, I got Desmume for that very purpose, since I wanted fossils and Heart Scales and Arceus plates so badly. For all the other processes in the game, I use No$GBA because it's faster.
The only catch to switching between emulators like this is that you have to copy the save files over. For tips on doing this, see my latest post in
this thread.
This is kind of off-topic, but when you go underground, save a game state (I love that feature of Desmume) every time you dig up a rare item, just in case the underground spontaneously stops working and your game restarts. (The save state saves you the trouble of leaving the underground to save.) I learned that the hard way when I had dug up several rare items, and all of a sudden there was a "communication error" and the game restarted, meaning I lost all those items...