Welcome to the forum...
I'd definately recommend going for the symetrical effect. EG: A Datamoon on the far left pointing right(ish), and the other datamoon, far right, pointing left(ish)....
Do the same with anything that you have two of....ignoring the obvious castration joke
Mirrorballs are indeed a pain in the neck to get working (due to the lighting angles etc), but do make an unbeatable scene setter for "slowies". (One gig I went to as a guest recently just left some of their barrel effects running during the slowies, but switched of his sound-to-light stuff...the atmosphere wasnt quite right....and a UV seems sooo static...
I use an "Apollo" which is one of those effects that several companies seem to have their badge on...I think mine was Eurolite, but Chevaut(sp?) make them too...they're like a black hollow sphere with a 300watt lamp "hidden" in the middle, and with around 70 multicolour lenses (all white lenses optional at time of ordering)
As for illuminating the ceiling, I no longer do this...as it seemed to make my bulb life disintergrate when I did it with Twister III's.. some bulbs just dont like being used upside down I guess...but ceiling projection of effects is OK if your effects have angle-able mirrors, so the bulb stays the right way up, only the mirror does the "upward" bit.