aznboi: Thanks for the educational on the various forms which proportional representation can, and may, take. I wish you'd expand on them to identify the respective weaknesses and strengths of each variation as in actual practice.
It kind of depends on the exact manifestation of a particular voting system as you can have variations on the same system (eg: open lists vs closed lists for Mixed Member Proportional (MMP)). Generally speaking though:
MMP
Pros:
-election results more accurately reflect the will of the people, so less votes are wasted
-still retains local constituencies that are elected under First Past the Post (FPTP) while party lists are used to correct the discrepancies that it always produces
-can correct the structural barrier of FPTP that leads to underrepresentation of women and minorities in the legislature
-corrects the problem of FPTP where the voter has to vote for their MP/MLA and party with one vote (they may like a candidate but not the party and vice versa). Voters would get two votes: one for the local representative and one for the party.
-reduces the regionalization that can occur with FPTP (eg the Bloc getting double the seats of the NDP even though the NDP gets double the votes)
Cons:
-more minority governments (pro or con depending on your viewpoint; more the most part, minorities are usually pretty stable under PR)
-related to women and minorities, even though it leads to more women and minorities in the legislature, you get an interesting phenomenon where you get them more on the side of opposition and less in the governing party
-the counting method can be confusing to voters
-depending on whether you used closed or open lists, you can get two classes of politicians: those loyal to the constituencies they were elected in and those more loyal to the party
Single Transferable Vote (STV)
Pros:
-eliminates the need for strategic voting, increasing choices for the voter
-requires that MP's get elected with a collective majority
-while proportional, fringe parties generally don't get elected, due to the high threshold needed as well as the need to garner support from voters of other parties
-helps to correct underrepresentation of women and minorities without the imbalance seen in MMP
-because of the use of the preferential ballot, it requires politicians to generally act more civil and respectful as they need to be able to appeal outside of their core support
-because of the high threshold needed to get elected as well as the fact that they'd also be competing against members of their own party, a great deal of pressure is placed on politicians to pay attention to their local constituency.
Cons:
-the counting method can be confusing for voters
-the constituencies are large, which may be unworkable in a country as large as Canada and can diminish the feel of having a local representative
-related to the politicians and their local constituencies, sometimes, they prioritize local issues over the national interest or party policy
There's probably more; but that's all I can think of for now.
I'm more in favor of a preferential ballot system. I'm pasting the answer I gave in the other thread:
Instant run off voting is certainly better than FPTP, I would use it to elect the PM as well. The only real weakness is the lack of proportionality, so I would still view it as weaker than STV or MMP; but that depends on your view of what a legislature's general purpose is supposed to be.