I personally wouldn't recommend a DStwo if you're going to be using it on a 3DS, especially if you plan on using Streetpass. I recently switched to a random R4 SDHC clone that I had laying around (now that even non-DSi carts are compatible with the 3DS), and it made me realize how absolutely horrid the DStwo's CPU is, in terms of battery drain.
With the DStwo, I'm lucky if my 3DS is alive by the time I get home (even if it's doing nothing but sitting in the cartridge slot), but with the R4, it can last several days in sleep mode. IMO, the extra CPU power isn't even worth it when you could just run RetroArch or VC for your emulation needs.
As for which one you should pick: I'm not sure if it matters at this point. As long as it'll run the latest Wood R4 or YSmenu, it should be fine. But I think your best bet would be the R4i Gold labelled with the website 'r4ids.cn' (the only one officially supported by Wood R4).