Multi-Domain and Domain Mapping Plugin differences?

Hi,

i'd like to undestand better the difference between Multi-Domain and Domain Mapping Plugin!
(sorry but my english is not so good)

If i undestood right the Multi-Domain create new domains that share the same site/bolg,
Domain Mapping instead help to assign a different blog for each different domain ... is this?

So i'm building my WPMU blog with subdomain and i have

domain1.com
blog1.domain1.com
blog2.domain1.com

and probably i need to have in future

domain2.com
blog1.domain2.com
blog2.domain2.com

with differents contents from domain1.com, but shared posts!
So for to share post in all Network i installed Post Indexer Plugin, now for to create and menage Domains and subdomains do i need to install both plugins Multi-Domain and Domain Mapping?

Another techincal question, to have lot of domains and subdomain in a Network can give some problem to the "navigation" (for exemple should it become "slow" or something like this)?

thank