What happens if I choose one Mii or another when linking my 3DS to the Miiverse?

I just finished updating my 3DS to the new 7.0.0-13 update, which adds the Miiverse to the 3DS and finally combines the funds of both Wii U and 3DS e-shops.

Right now, my 3DS is giving me the choice between two Miis to represent my person on the Miiverse:

• [3DS Mii]: Personal Mii for this [3DS] system
• [Wii U Mii]: The Mii currently linked to this Nintendo Network ID

I guess, if I choose the 3DS Mii, my Mii will change on my Wii U as well, due to the Miiverse being an integral part of the Wii U.

But what if I choose the Wii U's Mii? Will my Streetpass Mii change? Will I lose saves?

I found a fairly interesting piece of info on this site, the site's comment section to be specific.

HaNks said: #73

there is an oversight with the update, i just lost my wii sports club save. you need to overwrite either your NNID or 3ds system mii.

the problem is both of these miis potentially have save game data linked to them. i know of various 3ds titles that have this, so i didn't want to overwrite that one. i booted up my wii U and bang my profile was wiped. there will be more people losing mii-linked save game data after doing this.

HaNks said: #80

@sleepinglion only if you chose to replace your existing NNID mii with your 3ds system one, which was the first option. mine were both the same with the same name, but clearly save games linked to them differentiate, at least on the wii U side.

Unfortunately, this only describes what could happen if you choose to replace your existing Mii with your 3DS' Mii, but not what could happen if you choose to keep your existing Miiverse Mii.

I'm especially worried about how Streetpass and Miiverse coexist, since I don't want to use the same Mii on both.

If you choose your Wii U's Mii, your 3DS Mii will also change, but only in the way that it looks. Your Streetpass data will NOT change whatsoever (you can even choose to use a different Mii in Streetpass Mii Plaza, fun fact of the day). Nintendo is very secure in every way (as far as I know), so go ahead and change that thing! But, just in case, I'd back up or copy your 3DS's user Mii so it doesn't get overwritten (if you want).