I realise this question has probably been asked a thousand times before, but I've no desire to trawl through thousands of posts, so I'll ask it again...

I'm going to start devoting some serious time to making music, but with so many programs on the market, where do I begin?

I can find my way around Cubase quite easily, but up until now have only been importing loops from FL Studio, not actually using an instrument to create sound.

Should I concentrate on Cubase? Or is Ableton the way forward? What's the difference between the two?

No, you need not say any more as I quoted Minimal-Animal saying and I was only referring to his post. haha!

I'm with most other music makers... Mac and Logic (pretty much nothing else needed). The host plugins are top notch and it's super fast to get to grips with. Cubase is the closest you'll get on the PC but it's not quite as easy to handle straight away and you'll need to pull in all sorts of vst plugs to get it sounding good (same with Acid and Ableton).

Can anybody recommend some decent plug-ins to work with Ableton/Sonar?

this is good , and a great way for people to learn how sequencers work and get them away from using sample cds to make tracks which is easy and lazy

hobnox.com

choose blank . select your drum machines , synths and fx and plug all into mixer and output and get creative all online.

TBH, never used Ableton because Ive never needed anything else but Logic. Check out SFLogicninjas vids on youtube, he really gives a good breakdown of what is possible with logic. I just find it easier to use than anything that I have done so before (Cubase, Reason), and I use vector synthesis quite often on ES2.