Injected VC wads are great for N64. Wii emulation kind of annoys me. Not to sound too much like Chad here, BUUUUUUUTTTTTT.......
Yes, there is a delay in the emulators, and it's VERY noticable. Speed runs in mario for SNES or GameBoy? NOT A CHANCE! it's like a 50ms delay between button push and jump! I could MAYBE tolerate it for an RPG, but that's about it.
The nice part about the injected wads, is that the emulators are made by nintendo themselves. Some games run perfect, others... not so much. Super Smash Bros works great though! I think GBATemp's wiki has a
compatibility list for wad injections.
As for the N64 flash cart, it's 199 for the adaptor, and you'll still have to buy a neo flash cart for GBA to load your N64 roms on. I'm pretty certain there's an SNES version that operates in the same way.