AVID and FCP are the big boys.
They tend to be popular with broadcasters and film production houses. Programme makers are a conservative bunch so they hanging on to what they know best, which means Avid is it's various flavours.
FCP is making inroads, and the worlds biggest production house (BBC) is now shunting over to FCP, albeit chaotically. as it does essentially what Avid can do, for allot less money.
Both systems have their issues though. I started working with Avid when is ran on a Mac II, and was highly experimental. It's now a very sophisticated beast indeed. So too is the young upstart FCP, and both have a inclination for crashing.
Which is better?
well i dunno.

People like analogies, and often compare the two systems with cars. Why this is i do not know, as fundamentally they are about producing commodities, i.e TV programmes and films. Both systems are essential to know because that's what the industry demands.
Despite editing on Avid for the last 15 years, and though temperamental, my personal preference is FCP because it makes the WHOLE production process less hassle by sticking to one logic set. At Avid they seam to make it up as they go along, and Unity?!!!..... the less said the better.
As interments of sweat, toil and torture, Avid is nice and FCP is kinder.
N.B
On Rendering...
Both need rendering on output, so the realtime effects are 'a much of a muchness' if you configure both systems properly......... but when I walk away for 30++ minutes during rendering for the finish and final lock,
I know on my return with a cuppa in hand that FCP will still be standing.
