get the mp3 sway bar kit only after you've upgraded the struts. you really shouldn't upgrade just the rear for two reasons:
1) it will induce more oversteer than understeer, which means it can be dangerous if you've never aggressively driven a car that oversteers. the mp3 kit will create near neutral handling. i'd rather have a lil understeer on the track than too much oversteer in a FWD car.
2)the bigger sway bars will create even more stress than the stock bars on the wimpy strut endlinks. 19mm bars have been known to break the tabs off the factory struts.
i went with springs because i got a great deal on them. it was a small mod that really helps on the track and i love the look. struts would have made the bigger difference, but i'm only keeping my car for a few more months.
i say get some eibachs if you want a nice drop and a lower center of gravity. the ride is more pronounced, but i don't care. its not liek it rode like a bmw in the first place. i personally don't like "floating"