The issue you have here is that the risk involved due to hardware failure, coupled with the last of supplies and production that the manufacture has making a RMA a bit of an issue, could go poof in a second.
I personally if given the opportunity, would go with the singles because of their point of failure. There is a much higher chance that you may lose a whole rigbox due to a catastrophic event. In the case of the solos, you would only risk the solo unit in the catastrophic event.
There are other variables here you also need to consider.
Is your electrical at a fixed rate or do you pay via a tier? you will cross a user threshold or two.
Do you think either a Solo or Rigbox would last longer?
Can you even obtain the units?
The units do generate heat. It needs to go somewhere.
What you should really do is write up a spread sheet and two fact documents, one doc for every proposal. Then measure every potential risk you can see, and calculate the cost of remedy. That is how to mine. Then choose the least risky if any.
True that the singles offer much better insurance to your investment, but the difference lies in the outlay of cash for equivalent performance and space usage. Having 121 units might seem nice but considering that you will have to wire all 121 units to a untold number of hubs and those hubs will have to be plugged into more hubs so on and forth until you get all the units in as few usb ports as you can so you can ideally control them all from a single computer can you imagine all the cables? not counting where you going to stack all those 121 units? 1 or 2 might look cute and unobtrusive but 121 will take a lot of space when you consider the power cables that you will have to plug on all those units. And like I mentioned about the cash outlay since I was told by the company that they don't do multi-unit discount.... 121 x 599 = $72,479 while 2 Rig units will cost $59,780 or $12,699 less and with that money you can other an extra 21 of the singles and add another 17,472MH/s increasing your total output to 118,272MH/s vs 100,672MH/s for the same money.
Of course the main advantage to the singles would be gradual buildup, if you can't put out 30/60K down on a sucker like the Rig the singles are the way to go since you can spread the pain over time
Of course you will pay more for them in the end but that is the price of not being able to afford the big guns :p hehe