If you are buying a new 3-in-1 now, you'll probably get a 3-in-1+ regardless, unless the seller specifically says it is an old model. The old 3-in-1 is already out of production I believe.
To be perfectly honest try to get the old one if you can. The only thing the new one has over the old one is a bigger memory, which is totally pointless for games and is only useful if you want to play GBA videos (which, trust me, you don't). On the CycloDS the old one is better natively supported than the new one, so it's better to get at the moment, but if you can't get the old one that's fine. The new one still works for most things and for things that don't you can use GBAexploader.