FPTP V PROPORTIONAL VOTING

I only had enough time to post what I had found about the latest Welsh Opinion polls http://ukgeneralelection2015.blogspot.co.uk/2013/07/welsh-opinion-poll.html follow the link to read it all and see the source.

But one thing stuck out, which was the difference between a person giving their intention when it was for a FPTP system and so voting for the one they think will win, want to win or vote for to keep someone else out. Opposed to a system where within reason every vote counts in the proportional system.

UK General Election Voting Intention
Labour: 48%
Conservative: 23%
Lib-Dems: 8%
Plaid Cymru: 9%
UKIP: 8%
Others: 4%

National Assembly Election Voting Intention (Regional List Vote)
Labour: 25%
Conservative: 12%
Lib-Dems: 9%
Plaid Cymru: 23%
UKIP: 16%
Others: 14%

DIFFERENCE

Labour 23% decrease
Conservatives 11% decrease
Lib Dems 1% increase
UKIP 8% increase
Paid Cymru 14% increase

Now one factor is the difference between who represents you in one parliament opposed to a local assembly. But still just how much of an affect does FPTP v PROPORTIONAL have?
Does the feeling your vote will count make a difference? Will they then vote for who they want not who they do or don't want to win?

Your thoughts are welcome.