well, why don't you try www.sonyericsson.com and go to products>>>see all phones,,,
and you can also combare models with each other to know whuch is best,

there is another site that can help you ... www.gsmarena.com

higher model number should mean more functinality ,yes....but some times it is not that way....like P990 and P1...>> p1 is more advanced
or K750 and K550....>> K550 is concidered an upgrade for K750