Transferring the data wirelessly is the slowest method out of the 3 available. It could go anywhere from 30 minutes to 8 hours depending on how much GB your old SD Card contained and if your new microSD card is able to receive that much of data. There is absolutely zero reason why you should ever pick that option over the others. Avoid it at all costs, unless you have a very limited Internet.
Ironically, it's much better if you perform a Low-capacity transfer than the former option, as it won't transfer the downloaded titles themselves but the saves are. It's much faster if you download these titles back from eShop, assuming that you aren't about to hit your Internet Connection monthly cap.
If you happen to have the right screwdriver to open up the back cover of your New 3DS XL at that time, then the last method of transfer is the fastest out of the bunch. All you have to do is to copy all content to your target microSD card and a 30 minutes long transfer and you'll be done!