So to hopefully answer several questions:
1.) I believe the 2150 Battery is a little thinner (.5mm) than the Seidio 3500. I have the HTC 2150 and prefer it since it actually matches its claim for battery power. See this
http://androidforums.com/incredible-accessories/119578-battery-testing-results.html
2.) I prefer the extended battery for the freedom from the wall. I travel frequently, and sometimes dont have access to charge up my phone (I am an avid outdoorsman and camper). Knowing I can use the phone heavily and not need a charger immediately is nice.
3.) They do make cases now (Hard rubber and Silicone) for the HTC extended battery. See this
http://androidforums.com/incredible-accessories/177529-cases-htc-incredible-extended-battery.html
Hopefully this helps everyone.
Cheers,
-Sev