Well there were 2 318's and 2 325's available in the North American market. First being the 84/85 318i with an m10, same basic engine as in the 2002, approx 110hp. I've never personally driven an m10 powered e30, although i would imagine that it would feel slow in comparison to mine.
Next up we have the 325e or "eta" which is a 2.7l m20b27, 121hp 170lb/ft. Low-revving, high-torque engine that gets pretty good milage around town. I have one of these, and i love it, i can get 30+mpg around town, and the torque makes it very tractable. Best money you can spend if you do get an Eta is to buy a good chip, it really transforms the car. These were available from 85-87 and in 88 there was a model called the Super-eta which shared more parts with the "i" engine and in turn could rev higher.
Third we have the 325i with the m20b25, 165hp 164lb/ft. Higher revving, higher horsepower 6-cylinder e30. available from 87-91 in North America. Its a nice engine, but in comparison to the Eta it needs to be revved more to get power out of it. I prefer this engine the most out of all the available engines in NA, its the best compromise IMO.
Last there is the 318i with the m42, 136hp 134lb/ft. Available in 91 only (92 for convertibles). The m42 is the most modern engine available in the e30, likes to be revved. I have driven one of these and i found it to feel slow at anything below 4000rpm.
In terms of differences b/w the 4 and 6 cylinder models, the early 318s had a different rear subframe (drum brakes, smaller diff) and I believe the front suspension was different too. Other than that, the differences b/w the cars are minor, different differential ratios and trim levels. One advantage that the 4 cylinder e30s have over the 6 cylinder ones is weight. The 318s have less weight over the front axle, and in turn handle a bit better.
In terms of reliability, my experience has shown that the 6 cylinders are very reliable if they are maintained. The timing belt should be relaced every 50,000miles or 5 years. In the 2 years that i have had mine, i have had to replace the alternator once (the original lasted for 20yrs) and the ICV. Nothing too major. I would imagine that the m10 and the m42 are just as reliable.