I did quite a bit of research about long wires vs. dipoles before I put up a limited space dipole in my back yard. I only have about 70 feet from the back of my one story house and a utility (cable / phone) in the back corner of the lot.
I was going to put up a random longwire, what stopped me was that research showed I needed a GOOD RF ground. There is a lot of theory behind this (the ground acts as a "reflector" to complete the other half of the dipole, the long wire acting as the first half). The other way to explain it is that the feed for the long has to work "against" something to crank out a signal.
Simply connecting to the ground of the wall outlet is not enough as at RF frequencies, due to distributed inductance and capacitance, this is NOT really a ground and you get a lot of RF in the "shack"; i.e. the radio (and probably you) act as the other half of the dipole and radiate like mad :-)
Bottom line is you need a good RF ground to connect the outside of the coax against (center goes to the long wire) and this can be as many as possible and as long as possible radial wires, either buried or laid on the ground surface. The other good ground is 4 to 6 eight foot copper ground rods driven into the earth about in a circles 6 feet apart and bonded with heavy copper wire.
You then can feed the long wire (center conductor) and ground (braid) from a tuner and use the long wire on a number of bands.
The better solution (for me) was to string up the longest dipole possible (I ran it diagonally across the back yard from eave rain trough (about 12 feet high) to the utility pole (about 20 feet high)) and run the dipole center feed coax to a tuner (AT-11). Even thought I only have a 66 foot dipole the AT-11 can load it up from 80 to 10 meters, I just checked into a net 500 Km away with that setup on 80. Also worked a local 10M FM net the other night. It would probably be better if I fed it with ladder line and a balun at the tuner (less losses, as the VSWR on a non resonant antenna is high, and coax is more lossy with hign VSWR).
Do yourself a favor and get the ARRL handbook and read the section on wire antennas, or get the ARRL antenna handboodk. I also found a couple of good papers
"HF SSB Users Guide"
"Smart tuners for Stealth Antennas"
Bottom line, get as much wire up as high as possible, the dipole if probably better in most cases, if you have to use a long wire lay a lot of radials or use a lot of ground rods, and feed it with a tuner, ladder line being better than coax.