They're both very safe; the 700/940 is a more reliable car with respect to the rock solid powertrain. The 850 is a great car, but I'd shoot for m/y 1995 or newer.

With respect to 740 and 940, with the 91-92 740 vs. the 940 series, we're splitting hairs in station wagon form. The 940 has a few more options in 91-92, but the car is the same as the 740. The 92 740 is the same as the 92-93 940 wagon. The 94/95 940 has dual airbags and a couple of trim options (glass roof) that the 91-93 740/940 didn't have. The 940 supposedly had better insulation than the 740, but that was really only the 960, which is a much different car (except for looks). For all practical purposes, the difference between a 91/92 740/940 wagon, is the options and the seats. The 940 had power roof, the 740 (91-92) had manual roof, electric optional. 91 740 didn't have standard ABS (a personal recommendation of mine) but the 92 740 did. The 92 740 did have standard ABS.

Supposedly the entire 940 model line had a locking differential (traction measure). The 92 740 did in fact have this as standard.

Basically my recommendation would be to find a 92 740, or a 91-95 940 wagon. Avoid 16v models for maximum ease of maintenance and lower maintenance costs.

This is not as organized by model year as I'd like; I'm short on time but wanted to say a few things.
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Chris Herbst, near Chicago.