For your husband's flying it may depend on his company travel policy - I'd look at these points:
1. Do they have contracts with either OW or *A carriers?
2. What is the company travel policy for class of travel (i.e. all flying in economy unles he's middle or senior management; or perhaps under 3 hours in Y, over 3 in J)?
3. What cities is he likely to have to fly to (long-haul or short-haul)?
4. What frequency of travel (once per week/month)?
5. What type of tickets (point-to-point return or multi destinations)?
The first two points will inevitably sway his decision to having the majority of his flying in one alliance, then it's a matter of the answer to points 3 to 5 will help to fine tune the selection of program. He may end up flying on both alliances (as some flying to certain cities might necessitate, because of say flight times, travelling with one carrier) and thus joining one program in each.
Until we know the answers to these questions, it is hard to be able to categorically show which Alliance or program would be better.
Prior to knowing these, here is a bit of a preamble:
If he is going to fly mostly economy and on *A carriers, I would plump for United Mileage Plus. UA does have good earning rates on all *A carriers almost irrespective of fare class (unlike Singapore Airlines) and excellent (read low points required) for redemptions.
If mostly Business (or First) on *A carriers; then Singapore Airlines may be the best (given their PPS club for regular J & F pax on SQ flights).
If he is going to fly mostly lower-discount, high volume economy and on OW carriers (mostly QF or CX), then I'd plump for American Airline's AAdvantage and look to put him onto a Platinum Challenge (using your first flight MEL/SIN). It would depend on the fare basis of CX tickets bought.
If mostly high-discount, high-number of economy flights and on OW carriers (QF and CX); then the QF program beings to win hands-down. CX earning on all fares bases through the QFF program is excellent, whilst on AA is bad (only full Y class gets status).
If he is flying J regularly, then a couple of short-haul return flights will get him AA Plat; whilst it will take a while longer on QF. AA's Plat benefits are better (auto upgrades to J on mid-priced Y tickets, redemptions, etc) than QF's at present, particularly with the changes to the QF program effective 25 May 2005.
One strategy if h is flying very regularly in J (or F) on O carriers, then I'd look at getting both QF and AA. Start with putting all flights on QFF until he gets Platinum (hopefully before the end of the year). Then stop putting to QF, join AA and put him on a Platinum Challenge - one decent flight in J (SIN/LHR) will get him AA Platinum (using the Challenge) and as it is likely to be after 1 July 2005, he'll have Plat status until 28 February 2007. Then re-focus on the QFF in early 2006 until he re-quals and then put all flights back on AA Plat. He will be getting the best of both worlds, all flights are credited at +100% mileage.
Again, with your travel (and your children), it will depend on the class of travel you will pay for. It is likely to dovetail into the same Alliance (if not program) as your flights may be the cheapest economy and only your 2 y.o. will probably have a seat bought for them at present (but definitely join them up to the FF program as well, if they are allowed).
You don't want to be in one Alliance with him in the other if you can help it. If you do any travel together, you want to be able to use the benefits together.