STV vs FPTP
STV provides proportional representation (PR) where by the percentage of votes is roughly translated into the percentage of seats. This is fairer than first past the post. For example in the general election 2010 in the constituency of Ayr, 47% voted labour but collectively 43% didn’t vote labour. This means that more voters wishes will be represented and less wasted votes
In addition it promotes voting within parties with the ranking system of voting. Instead of the traditional marking of an ‘X’ instead voters rank candidates in order of priority. On the one ballot paper there could be two Labour candidates and you rank them in order of your preference. For example in the ward of East Kilbride South in the upcoming council elections you would rank Archie Buchanan (SNP) 1st but then rank Douglas Edwards (SNP) 3rd due to his lack of presence within the constituency. [the only reason I have used this example is due to the fact I live in East kilbride, go find an example for your area as this will impress markers]
It also allows smaller/third parties to gain seats in the government due to the PR system of voting. For example the Green party saw an increase of 2.3% between 2003 and 2007 (introduction of STV). this is because more voters will vote for these parties where as probably wouldn't in the 'X' ranking of one party (they also saw an increase last week in the 2012 local elections too: went from 0 to 8 seats between 2003 and 2007 and then 6 more between 2007 and 2012)
However it has its disadvantages. One such disadvantage is that the ranking of candidates confuses voters and as a result in 2007 there were 100,000 wasted votes.
Another disadvantage is that, in theory, with AMS being a form of PR it should increase the representation of women. However this was not the case in 2007 as representation actually fell from 23% to 22% (2003 to 2007).
Similarities and differences
MMP and AMS are very similar to DPR Voting in many respects, and have the common goals of electing Single Member Constituency MPs as well as achieving a form of Proportional Representation.
MMP/AMS are known as Mixed Member electoral systems. Some MPs are directly elected as constituency MPs, some are elected by the party list method, which may be a regional list..
In MMP and AMS sufficient additional MPs are appointed from Party Lists by a calculated method (the method can vary slightly) so that the expanded parliamentary parties have approximately the number of votes that reflect their PR share.
DPR Voting is a 'Single Member, Party Proportional' electoral system.
All MPs are directly elected as constituency MPs. One MP for each constituency.
DPR Voting achieves Party Proportionality simply, as follows. The Party vote in the General Election determines the 'proportional number' of votes each parliamentary party should have in Parliament. These votes are shared out amongst the elected members of each parliamentary party. All the MPs of a particular party will have an equal share of the Party's total number of votes.
The main consequences are
1 With MMP there are two types of MP, those elected as constituency MPs and those elected from the Party List. Election from the List is by an agreed formula. The closed party list system reduces voter choice in favour of party control. Parties choose the candidates and determine their priority order on the list
With DPR voting, all MPs are directly elected as constituency MPs *. There are no Party List MPs.
2 When changing from FPTP, MMP requires either fewer larger constituencies or a larger number of MPs in the parliament (to accommodate the Party List MPs who do not have a constituency).
DPR does not require constituencies to be redrawn. The system works with the same constituency boundaries and the same number of MPs.
3 With MMP you may have overhang' seats. 'Overhang' seats occur when a party wins more constituency seats than it would be entitled to from its proportion of (party list) votes. 'Overhang seats may be managed by increasing the overall number of MPs. Different rules apply to different systems for dealing with such a situation.
With DPR Voting, the equal sharing of the Parliamentary Party votes amongst the elected members (resulting in a decimal vote value for each MP) means no extra MPs are necessary. *
4 With MMP a medium sized party can have MPs elected by the closed party list system to the parliament without contesting any constituencies. (unless the allocation of list seats to a party is made conditional on winning a constituency)
With DPR Voting, to be elected, all MPs must fight and win a campaign in a constituency. There is an imperative for every party to win at least one constituency in order to exercise its full Parliamentary Votes. If a party does not win a constituency but does exceed the voting threshold it is limited to one MP with a single vote (*Automatic election).
5 With MMP, the makeup of the parliament is a combination of constituency MPs and Party List MPs, the whole broadly reflecting the party political balance in the country.
In DPR voting the party political balance is achieved for votes in parliament because MPs have an equal share of their Parliamentary Party's votes. Similarly to FPTP, all MPs are locally elected constituency MPs and they have a responsibility to represent all their constituents equally on apolitical issues.
* except in the case of Automatic election', where a party does not get any constituency MPs elected but still manages to get enough Party votes to exceed the agreed threshold. In this case, the party concerned qualifies for one MP (the Party Leader) to be elected to the parliament as an MP without constituency.