Welcome to the forums, guess I`m the first one to reply.
I`d say go for it now, if you don`t have low download limit from your ISP, and won`t mind downloading twice.
If you start playing now - all your progress - including all achievements, will transfer to Steam automatically, once you install the game there (anytime after April 1st) using the same CD key.
As for the price - it`ll be the same on Steam as it is in our store, plus or minus 5$ - as it`s not set in stone yet. As a bonus from getting it from our store you get two booster packs - several rare Dwarf units and epic Haunter Dragon unit - about 20$ value.
And as I mentioned - if you buy from our store and start playing today - you will have to download the game again once you decide to install on Steam.
Also - our store is not the only place to buy the game - it`s also available at Impulse, Desura, Gamers Gate e.t.c although they don`t give away free units.