I'm getting Black because there is Black City with extra trainers, where as White has White Forest that has extra Pokemon to capture.
It really depends on what you want. More trainers or more available pokemon to capture.
Finish HeartGold and then buy and start Black/White. They might sell out for this month, but you can still get Diamond/Pearl/Platinum/HeartGold/SoulSilver at pretty much any store. They are stocked up, and Diamond and Pearl were released years ago.
I would guess Black and White will end up the same.
Just keep having a crack at the Elite 4. If you don't beat them the first time, then you still get the experience points from them and can try again.
Also, don't be afraid to use hyper potions, revives and full heals.
I bought 50 of each the first time I went through and came out of them with about 3 of them.
The Elite 4 will abuse full restores and so should you.