My advice to you is to wait. Dev preview could then be out, and you could keep your data. Less risk, plus, you avoid some of the bugs that remain, that are now being fixed. One question though, why would you potentially lose the data? How does your friend's way of pushing it to you differ from the way I would receive it by normal means? Surely they don't write the program to add a new update in the week after it leaves the core team? Surely that part would already be in place?
So, I actually wouldn't get the 8.1 that will be RTMed. It is the Microsoft development version, and has features like reporting bugs that the public won't get. I loose my data because it has to have a factory reset to remove that once it goes to the public. Also, dev preview is out in a few weeks, around April 14 - 27. Sorry for the huge range, just confidential the exact date.if you want to know mare, check out this app and the companion for Lumia Black, if you don't have it already. Keep asking these questions!
8.1| http://www.windowsphone.com/s?appid=5f5fc7e1-07a4-4551-ba86-7de24b6c7ee3
Black|http://www.windowsphone.com/s?appid=c67d7679-9017-47f9-9d1a-7effcafa58d0