Then there were 2

of the two, I would choose ARC

however, I much prefer the Quicksilver Mono 120's, and think this:

Pair of Quicksilver's, with KT150's is under $5,000. 

ARC is $10,000.

I would wait forever for the Quicksilvers, and use something else in the meantime. However long you wait, it is but a speck of time.

Two mono amps can be moved by one person, more placement flexibility, different cable lengths, ....

You could buy a used pair of mono blocks now, use them, sell them when the Quicksilver finally materializes.

elliot that was my preference as well, however they are hard to find used. QS has discontinued them. I agree with your points. The Ref75SE is not too heavy at 47 lbs.

@jimbones What loudspeakers are you planning to use and how big is your room?

I checked, I also saw no used quicksilvers, that tells you something!

My job designing corporate office space, projects typically 18 months start to finish: I told my employees part of their job was to keep me from compromising under pressure. It's always a war against the forces of compromise.

Not that ARC is a compromise as much as: In 3 years, what do you want to wake up and see/hear in your system?

I meant buy anything interesting used or new that you are confident you can re-sell when the Quicksilvers FINALLY arrive. 

For the difference in price too many options exist,

pounds is usually transformers, heavy oversized parts. two quicksilvers at 82 lbs or 1 ARC at 47 lbs?

Personally I like the look of the quicksilvers more, and, think thru the use and related costs of existing or new cables: the different lengths involved if monos or stereo.

it's a fun problem to have, best of luck deciding.

My reaction is exactly the same as @atmasphere . What are the speakers will you be pairing?