i am thinking of finally ditching my D3000 with 18-55 non-VR and 55-200VR (all Nikkors) for a Canon
Thinking of this, and also getting some sort of prime (either the sigma 30mm or canon's own 50mm)
Will it be worth it? Also, will it be better to replace the 18-55 from this kit by a 17-50 or something like that? All help is appreciated.
Between Canon/Nikon in this price point, you won't see much of a difference. The best way to describe it is that they're both different. Canon has some lenses that Nikon doesn't have and vice-versa. Right now, Nikon's entry level cameras have better sensors than Canon, but I'd expect Canon to release something relatively soon. In general you're paying more for Nikon right now (or getting fewer features). D3200 is similarly priced to T4i deals, and D5200 is around $900 for the kit which is pricey IMO.
In the end it doesn't really matter if you have Canon or Nikon, they'll both perform well.
If you want a very good walk-around lens, then the 17-50 (Tamron non-VC or pony up for the Canon or Nikon equivalents at around $1k) are very good. Personally, i'd rather get a few primes.