Hey man,
I own both... get the 4.2a as your first, it's not that much less bright than the 5.6 and is much easier on your equipment, less heat, better runtime, etc. With the 4.2, you can use a McClicky modded Z41...
At 5.6a, things change and you are looking at a zero res mod in the tailcap.
Don't underestimate the whole package needed for this quad, you'll need a solid host that's bored for 18mm cells- the solarforces will do, but the swicthes may not take it... so, an upgraded switch like a McClicky, and I highly recommend using IMR 18650s in this. You may want to work your way up to a bored out 6P over time, or an Oveready TL65 with a P60 adapter... verrry solid for good heat transfer. Also, be sure to snug fit the drop in with aluminum foil.
Other than that, you will, and I mean really will, be impressed.