Analyze the earn/burn rates for the routes/classes you fly to earn and what you want to redeem. Cost and availabiility are dramatically different between FF programs and the latter might be the most important consideration. Diamond used to be worth its weight in gold with many informal benefits not stated in the plan. Has this stopped being the case? When I checked years ago the BA/QF/LA/AA equiv. status was significantly less valuable than Diamond because of this, but of course only applies on CX. AA has seductive earn/burn rate, but you have to wonder about the future of AA, with "merger" with USAir quite likely which could severely devalue Aadvantage.