Well since they don't make a 600w MH you'll be using a conversion bulb for the 600 any way you look at it.

I'd say go for gold, get the 1000w digi (Lumatek are good) and simply use witch ever bulb you want.

Of course if you're hurting for money than you should simply spend the money on a conversion bulb for the 1000w hps you already have.

Hope this helps.
Bob.