Widescreen [or more accurately, OAR] 110%.
Ebert and Siskel did a special on WS vs FS some yars back, very illuminating; they showed both action flicks like Indy or Star Wars, and non-action films like The Graduate.
However, in some cases 'full frame' [NOT fullscreen] is the way to go--I believe Stanley Kubrick filmed most of his films in fullframe or open matte or something like that. Also, many Troma films are 4:3 because as Lloyd kaufman says 'Most of our films will be seen on someone's TV screen, not a movie theater.'
Even when I had a 4:3 tv, I bought widescreen/OAR. Now that I have a 16:9, it's even more important. Even though not all 'widescreen' films are in 16:9.
On 4:3 content, sometimes I watch it sidebar, sometimes I stretch it [*never* zoom]. In both cases, you're getting the full image; unlike Fullscreen or Pan and Scan, you are losing image 99% of the time.
Check out
www.widescreen.org for some painfully obvious examples. There's also a guide to anamorphic and widescreen on the Digital Bits.
Most DVD players or widescreen TV's have a 'stretch' mode [my tv has 4:3, Zoom [horrendous], Widescreen [not bad] Panorama [pretty good]].
About the "black bars": Ebert said something intersesting on that episode. If you see the bars, the movie must not be very good. You should be watching the movie.