To Buy Points or Not to Buy, that is the question

This South African trip has me stuck.  I need to secure air tickets before US Air does anything with their pending merger to American Airlines because one of the sweet spots in their award chart happens to be South Africa and I hope to take advantage of that before it goes away for good.  To get to South Africa via a US Air Star Alliance partner I need 70K dividend miles RT for each economy seat or 110,00K RT for each business class seat.  The business option has a lie flat seat and being this will be the 6th longest flight in existence a lie flat seat might be enough reason to fly business class.  Problem is I only have 156K US Airways dividend miles points.  I need three seats total.  156K points are good for 2 RT economy seats with some points to spare.  One business class and one economy seat  are 180K dividend miles currently leaving me with a 24K shortage.  There in lies the problem.  I can't determine which option to utilize.  Ultimately, I would love 3 business class seats but given that I hope to ticket the seats next month I won't be acquiring enough miles with all my tricks for three buiness class seats  (330K dividend miles) so one of us  is flying economy and they may be riding with a partner if I can't figure a path to follow.  I also have 63K United miles (another Star Alliance partner) but have a 17K point shortage for a RT economy seat booked thru United (they require 80K for a RT to South Africa).  It all has to boil down to my ROI (return on investment).   Economy seats for the time period I'm considering are going run $1900 each and the business class seat about $6300, thats about $10,100 in airfare and I'd be paying a little less than 10% of that to purchase the points needed to make up the shortages.  BUT, if I go for 2 business class seats and 1 economy seat I'd be getting $14,500 worth of airfare for about an 12% investment.  It's all a little overwhelming as both United and US Air have bonus points promotions running right now to go along with points purchases.  So for the next few hours I'll be caught up in this triangle of which seating type to select.  I just wanted to share with my readers some of what is involved in the thought process behind acquiring ticketed award seats.  There are many more options available to me but for various reasons my choices have been narrowed to Start Alliance partners and by a full accounting of the various points I have on hand.  Stay tuned as the decision will need to be made soon.