Your HTC Tattoo most probably is a SEA set and probably comes with SlideME instead of the regular Android Marketplace. To get the Android Marketplace you will have to root your device.

Normally when people mean "root" they mean installing a custom ROM, but true. Rooting gives you superuser access so you can flash a custom ROM.

First of all I don't think that the Pros and Cons for Android OS has anything to do with the Android Marketplace.

But I'm going to list them anyway.

Pros:
Open-Source
Android Marketplace (50,000+ apps)
Simple interface for daily basic tasks

Cons:
Rubbish battery life
Somewhat complex interface for more advanced tasks

SlideME's icon is a picture of the alphabet "S" while the Market's icon is of a picture of a Bag