Good article here on the Beeb, with some examples of what the different systems would have resulted in the 2005 election.
http://news.bbc.co.uk/1/hi/uk_politics/election_2010/8644480.stm
Pay particular attention to the Alternate vote example, and see just how unproportional Labour's preferred version is, and how much worse it is than FPTP.
You can also clearly see why Labour don't want PR, and why, despite making manifesto commitments 3 elections in a row to reform the system, they haven't done so. Especially AMS and STV show pretty much all their losses going to the lib dems.
Is that really a better prospect than the Tories for a coalition? Labour only want PR now because they have been decimated, and they still consider the social democrat side of the lib dems to be an extension of their own party...