Right, I understand saves can't transfer to the cart. There's no reason to use classic mode if I can make a backup and play it from an SD card right? So that's what I was getting at. Since I don't have any experience with this. If I do whatever the guide says to get a header now, and then later use the header of my own personal cartridge, will that screw with my save data/online data? Specifically talking about Smash Bros. The reason I'm not interested in using classic mode is because it's so much easier to have my games on an microSD card rather than carry individual cards around.
Now about dumping a save. So if I have the cart and then make a backup of that cart, is the save still able to go back and forth (gatewaymode and classic mode) since they would be the same header? Is that what you meant by wait until I have the physical form and dump the save?
Yep you can use your own dumped game on Gateway, along with the save. If you start now on a scene rom, you will have to stick with this save. I'd say just buy it (If you wish to support Nintendo), but continue using the scene rom and save. Especially if you want to start now. You can backup the .sav file to a site like Dropbox along with the header you used. That will ensure that you'll never lose access to your Smash save file.
Your other choice will be to wait for the game, dump it, rename to .3DZ and put on your micro SD and done. But then the cartridge will become useless. Just know that it's possible to transfer the save BACK to the cartridge, and vice versa.
I can see why you wouldn't prefer classic mode, it's a pain to use since you need the gateway card inserted to boot into classic.
And yes the save made from classic will work with gateway mode, being the same header.
Just think as the scene rom as it's own cartridge, different from the one you're gonna buy, each tied to their own save file.
It's really easy to add a header to the scene rom, just dump your own header from ANY game of yours, use Gateway Rom Patcher, and apply it to the scene rom. Allowing the program to fix the header for you when it prompts you. Rename to .3dz and you're done.
I personally use my very own header from Mario Kart 7, a game I won't sell or give away. I use this same header with all my Gateway online games, works every time.