Amex SPG may be a good start. You can transfer SPG points to different FFPs at 1:1 ration (Not UA). Check out Webflyer Converter for details. With every block of 20K transfer, you also get 5K bonuses. This should get your international travels covered.

I don't fly WN so don't know much about it. I do know they don't fly internationally and don't fit my travel patterns/destinations.

If I were you, I probably would concentrate on one (major) alliance and try to achieve elite status, so you may enjoy the perks of flying byond WN World.