Trouble with the 19" widescreen models is they're all TN+Film which means poor viewing angles (plus poor black depth and they're not true 16.7M colour), which means the screen only looks right when you're sat at your PC straight in front of the monitor. If you want something that can be viewed from round the room, the 20" models are much better since they're MVA, PVA or IPS panels with far better viewing angles (although still not perfect).
Of course you pay extra for the better panels in the 20" models though. They start at around £280 whereas the 19" widescreens start as low as £126. Huge difference.
To answer your question though, there's no benefit from going 19" apart from saving money, and if you go 19" you do get a worse quality screen. That doesn't mean the 19" widescreens are awful though, a lot of people would be satisfied with them, especially if you don't mind sitting at your desk to watch movies.
