Given the choice, solid every day. We have solid here (primary home) and, in 16 years, no sagging problems. The doors at our lake house are hollow and I hate them. No sound insulation and, as you said, cheap.

Solid doors can be a one solid piece of wood (rare) or doors with a particleboard center. Conduct your research as the costs vs benefits are significant. As the average lifetime of house renovations currently seems to last 20 years, hollow doors are a cost factor worth considering if you want to spend money on other features in the home.

If you are concerned about quality; solid wood doors.

Depends on whether you want to stain them or paint them. If painted, wooden doors show small unfinished cracks due to expansion/contraction of the wood. We frequently use what are called solid core doors if they are to be painted.

My answer for paintgrade doors would be neither; depending upon budget I would point you toward Masonite ( http://www.masonite.com) or TruStile http://www.trustile.com) doors. A solid Masonite 6 panel door (common in Connecticut) is 130.00 prehung. A TruStile is probably in the 300-500 range. TruStile is made from MDF but built like a traditional door with rails and stiles. A Masonite door is a slab who design/panels are the results of compression. Either way no hollow core and unless staingrade no solid wood. Either door will not sag if properly installed. Solid wood is going to move and you will not be happy. Not sure if you have a builder or architect but they should be able to explain the difference and point you in the right direction. Either that or go to your local lumberyard - not big box store.

I personally prefer square hinges, and if you are worried about weight you can upgrade your hinges, especially for high use doors, to ball bearing.

Best of luck,