Difference between Suica Pass and Pasmo?

Two questions: What is the difference between the Suica N'Ex Pass and Pasmo? Can I buy either one at the airport? Also, when riding the subway, if I want to change to another subway company's line, is the station right at the first line. Say I'm in Ginza, and want to change company lines, do I have to walk down the street to find the other station or are they together? Thanks.

Suica&NEX is a package that consists of a Suica prepaid card and a ticket for the Narita Express airport train:
http://www.japan-guide.com/e/e2359_suica_nex.html

Pasmo is a prepaid card similar to Suica:
http://www.japan-guide.com/e/e2359_002.html

Right now, Suica has a larger coverage area than Pasmo, but from spring 2013, the coverage areas will become identical.

Can I buy either one at the airport?

Yes.

Also, when riding the subway, if I want to change to another subway company's line, is the station right at the first line. Say I'm in Ginza, and want to change company lines, do I have to walk down the street to find the other station or are they together?

It depends on the station. Sometimes platforms are side by side. Sometimes nearby. Sometimes in a separate building. As for Ginza Station, it is served exclusively by Tokyo Metro. There are ample of signs that indicate where to go to reach which platform. Sometimes they include walking distance.

First of all, the issuer is different.

SUICA is issued by JR East. Pasmo is issued by the association of subway and private railway companies in Kanto region around Tokyo.

Now SUICA and Pasmo can be used interchangeably in Kanto region. So which to buy doesn't matter as long as you stay within Tokyo.

NEX SUICA package consists of SUICA card and Airport Railway ticket from/to Narita Airport. NEX or Narita EXpress is the name of the airport express.

Today new information is released that SUICA, ICOCA, PASMO, PITAPA, TOICA etc. can be used interchangeably from MARCH 23, 2013.
http://headlines.yahoo.co.jp/videonews/jnn?a=20121210-00000024-jnn-soc...

Suica is the pioneer among many fare-collection systems including PASMO, ICOCA, etc.
It was first introduced by JR-East.
Later Tokyo Monorail and TWR (known for Rinkai Line) began to issue Suica Cards.
The following things are common among IC cards used on these systems.
: The cards can save time for handling cash and purchasing paper tickets.
: The cards are not rail passes for unlimited travels.
: The cards do not give a discount in particular.
: A same one card should be used to enter and exit a network. (If a card has an entrance record but does not have a corresponding exiting record, it can be rejected at the next time.)
: Basically the systems do not collect charges in addition to base fares; a passenger should pay separately for the charge in order to take a JR express train, a Keisei liner train or so.
: A card cannot be used over plural networks which are not connected. If you have entered Suica-PASMO network but have not yet exited it, you cannot enter ICOCA-PiTaPA network in Kansai with the same card.

Say I'm in Ginza, and want to change company lines, do I have to walk down the street to find the other station or are they together? Thanks.

That depends on a combination of stations.
Tokyo Subway Route Map with station codes
- http://www.tokyometro.jp/en/subwaymap/pdf/routemap_en.pdf
Roughly speaking about the subway in and around Tokyo, those located near in a same transfer group on this map are rather easily accessible to each other.
If you mention a particular case, the situation can become clearer.