Crawl everything under the hostname for each start address VS Only crawl the Site Collection of each start address

for more details go through below post regarding sharepoint 2010 search
http://suryapulipati.blogspot.com/2013/04/sharepoint-2010-search-and-scopes.html

http://spdev5:8004/ (root site collection)

http://spdev5:8004/sites/site2   (sitecol2)

http://spdev5:8004/sites/site3 (sitecol3)

Crawl everything under the hostname for each start address	Only crawl the Site Collection of each start address
You can go for this option, If you want to enable search for all site collections under one web app.	If you want to enable search for only one specific site collection
You have to create a scope to filter sitecol2 and 3 results in the root site collection.	You don’t need to create scopes If you are crawling only one site.
It will crawl all site collections under this site when add to the content source. You can use “Local SharePoint Sites”. http://spdev5:8004/	It crawls only root site collection not all site collections like sitecol2 and 3. You have to create custom content source http://spdev5:8004/
To filter results in the root site collection, you have to write rules in the scopes like this. Include http://spdev5:8004/ Exclude (Sitecol2) http://spdev5:8004/sites/site2 Exclude (Sitecol3) http://spdev5:8004/sites/site3	No scopes needed. Include http://spdev5:8004/
Create custom page in the root site collection and add “core results web part” to this page. Enter scope name in the web part scope property.	No need of custom page unless you are crawling more site collections.
	In future if you want to enable search for sitecol2 or 3 you have to add the site to custom Content source.  http://spdev5:8004/sites/site2 When you crawl this site, you can see site collection2 documents in the root site collection search results (if you  have permissions on site2 or 3). Now scopes will come into the picture again. Include http://spdev5:8004/ Exclude (Sitecol2) http://spdev5:8004/sites/site2 Exclude (Sitecol3) http://spdev5:8004/sites/site3
So when you want to go for enable search for multiple site collections under one web app, better option is “Crawl everything under the hostname for each start address”