There is no way to find a Manectric wild in Pokemon White. You can find it by:
A. Evolving Electrike (Which is also unobtainable in 5th generation)
B. Transferring it from DPP or HGSS
Pokemon black has exclusive Pokemon and an exclusive location called black city where you can battle trainers with high leveled Pokemon and you can catch reshiram, while for white version it has more exclusive Pokemon, a exclusive area called white forest where you can catch lower leveled Pokemon of the trainer's Pokemon and you can catch zekrom.
One of the only differences of the games is that they both have an exclusive area. Pokemon Black has Black City, which is full of trainers to battle. Pokemon White has White Forest, which has both trainers and wild Pokemon that are exclusive to White. In other words, White version has more kinds of Pokemon, so I'd say it's your best choice.
No.
Nowhere , you see, Eevee is not in the new one where black and white takes place. Eevee is in the Kanto region so it's not in Black and White.
there's no elekid
You get it the first time you go to the resort area.
at black city area and route 18
it is the area before the bookshelves in the second gym
Sigilyph can be found in the Desert Resort area in Pokémon White.
As with previous Pokemon generations, the basic gameplay is the same, but there are some key differences between versions, a little more so than those previous games. 1 - Like all Pokemon games there will be some Pokemon which you can ONLY get in one version or the other, including the mascot Pokemon Reshiram in Black and Zekrom in White. 2 - After beating the game you gain access to a new area where you can catch older Pokemon for the first time in the wild. In Pokemon white this area is the White Forest and in Black it is the Black City. 3 - The 8th gym leader is different based on which game you get. Both are Dragon trainers but if you play White you face Iris and in Black you face Shaga.
Tauros can be found in the Safari Zone usually in Area 3.
it means that rare Pokemon are in that area. these Pokemon cannot be caught anywhere else in the unova region.