Liberal Democrats and first past the post

So what if Labour is pushed into a poor third place and the Liberal Democrats come a close second to the Tories?
How will proportional representation then look to the Liberal Democrats? If as is possible, Labour self-implodes it is the Liberal Democrats who stand to gain the most at the next general election. So much so that under a first past the post system the Liberals would then have the ability to challenge the Tories for outright power!
On the other hand proportional representation at the next election would leave the door open for the Tories to do deals with other parties, leaving the Liberals stuck on the back benches.
If there is a hung Parliament, the Liberal Democrats might like to think twice about having proportional representation for MPs at the top of their negotiating list.

Hung Parliament?

If there is a hung Parliament where the Liberal Democrats hold the balance of power... which of these three options would they prefer?

Work with the Conservatives under Cameron, or
Work with Labour under Brown, or
Work with Labour under another leader?

If it were to be the latter, and with the polls pointing towards a hung parliament, it could be said that a Liberal Democrat vote was a goodbye to Brown vote. How would that go down with the voters?

It makes one think.