from what i've been told you should opt for a lcd over a plasma,as the picture quality is better.
As with anything like this it is pretty subjective and dependant on how much you are spending.
It is traditionally concidered plasmas are better for screen sizes over 32 inch, and they give much better black contrasts than LCDs. Of course a disadvantage is that they are prone to screen burn and generally may not last as long as an LCD - though screen burn is much rarer in new plasmas.
But LCDs are easily catching up and the line of which is best is blurry to say the least. I 'd guess plasmas will lose out to LCD in the long run.
Then you have the whole, do I wait and get a 1080p screen when they become cheaper or will 1080i do? From what I have read it really makes little difference from distances of 5ft or more.
So I suggest you go to a proper TV shop (not comet or currys!) and get a demo of each of the TVs you are interested in and watch both a HD source picture and a SD (normal) picture and make your own mind up based on what you see. Don't be suprised if standard Sky looks worse on a 42" plasma/lcd than it does on your old CRT TV either.
Oh and I say not currys or comet for a demo because generally the set up they have there is not optimal! Of course I didn't follow my own advice.....
My only hope is that Sky see sense and drop the £10pm extra charge for Sky HD.