Those are what the 2 setups it should come down to. Good choices.
For your first setup the a51 will allow you to push their coverage and cover more possibly because they can move independent. Their coverage is advertised as 3x3, but to compare to a 1000hps, I would say no more that 2.5'x2.5'. Same with at200, which there are 4 of in a at600(as I'm sure you know by this point). The at600 will cover the 4.5x4.5, but maybe not as great as the smaller units spread more.
Also if your going to do 4 a51's, I would do the new SGS's. I know it's a new light(sort of), but I would guess that it will perform possibly the best of any a51 to date.
Once/if you upgrade to the 4x8, the at600's would be epic, and my choice. 2 of them over a 4x8 is the perfect situation. Perfect coverage plus some crossover going on. Having one unit is a big convenience over multiples, and the AT is designed to match the 1000hps in that space, so your not loosing coverage like with some other higher wattage led units.
If you have the 4x8 and you like the apache's then you should.
Otherwise maybe get 4 a51 to start and then 1 big AT when you upscale. Best of both worlds. Eventually AT center and then 2 a51 flanking each side.