As I mentioned before, SAMSUNG T809!!!
http://www.samsung.com/Products/MobilePhon..._T809OKATMB.aspI LOVE that phone.

My sister had the Sidekick for a while.. It's pretty cool. I think it's worth it. The A900 is supposed to be a pretty awesome phone, but it depends on what you want.
With a Sidekick, you can go online and IM people and stuff. It also has a camera, right?
A900 has TV. But you have to pay extra for that..
Service charges vary though..
I don't exactly remember how much money it costs to go online and IM.. Sidekick: An extra $20 per month for unlimited internet access (AIM, web browsing, email)
They also have different service plans. Sidekick is for T-Mobile, A900 is for Sprint.