I own a Nintendo 2DS and I frequently download updates from the eShop. There is an option to download now or download later. If you choose the latter option, the 2DS downloads in sleep mode. I usually just use download now, but when I do use the download later option, there does not seem to be a difference in download time. Is there an actual difference in download time between these two options?
There will be a very small increase when downloading in sleep mode, because your system is not using as much internet as it would be if it was open (the more web pages open, games playing ect., the bigger this difference will be). But as I said this is very minimal and not really noticeable, so the short answer is no.
It just depends on if you want to carry on using the system for a while and download later, or if you've got time to download it now.
No. If you download in sleep mode, it is simply downloading when it is in sleep mode, vs. not downloading at all.