Agree with what Jenbel says, but I would say that you have to speculate to accumulate. The purpose of Frequent Flier points is to encourage brand loyalty.
If you are not prepared to be loyal to the brand, as you state "when I do fly, ticket price takes precedence over carrier", then it will take you a lot longer reap the rewards that the loyalty scheme offers.
I am not saying that you should blindly book Virgin no matter what the cost, but my experience is that nailing your colours to the mast of one brand - be it an airline or hotel chain, does pay huge dividends in the long term, if you are a regular traveller.
The advantage of the Amex offer that Jenbel has pointed out, is that the proper Amex Membership Reward points can be converted to a number of different schemes, where as the pseudo Amex card you have at the moment is administered by MBNA, and the points you earn are awarded as VS points.
Nick