BMW 318ti or 91 300zx?

I really don't consider those two vehicles to even be in the same class. Sure they're both RWD 2+2s, but the 318ti is the lowest end of BMW's American offerings in that era, where the 300ZX (especially in turbocharged form) was Nissan's top model. The Turbo Z has more than double the power of the 318.

That said, here's my opinions on the different areas you asked about:

Handling: Toss-up. The 318ti (and E36 platform in general) is very popular in the autocross world because it's a good base and easy to make handle amazingly. The Z will probably handle better factory stock though.

MPG: NA 1.8L 4 banger versus a twin turbocharged 3.0L V6. Obviously the BMW will be much better.

Reliability: The BMW has a few well known failure points that as long as they're well maintained it will last pretty much forever. As long as you do your own maintainance a 3 series isn't much more expensive than an average Ford to own. I own an E46 325i and have only spent about $100 more in the last year than I did on my old Crown Victoria.
The Z I'm unfamiliar with, but have not heard any major complaints about them. I would assume the situation to be similar to that of the Acura NSX, where anything that's shared with other vehicles will be cheap to work on, but the special parts only for your model will be pretty crazy. Check some Z forums before taking my word for it though.

Looks: Obviously very subjective, but even as a fan of the E36 BMW platform's appearance over all I never liked the ti model. The hatchback setup just looks like it was cut off and there's supposed to be more car back there. The Z wins hands down on this one in my opinion.

Overall better: Depends on what you're looking for. If you want a daily driver that you can trust as your only car but still have some fun tossing it through the curves, I'd go for the BMW. If the prices are similar and you're looking for a second vehicle that will just be all out fun, I'd go for the Z.

For safety reasons, I would choose a bmw. The 318ti is more reliable than the 300zx although you may have to afford a higher expense on the repairings.

300zx was a supercar in its day. the 318ti was a good sporty car. They will both handle great, get decent mileage, be horrifically expensive to repair. The 300zx is a better looking car in my opinion. Personally I would not want to pay the repair bills on either... but For an occasional weekend driver, I would prefer to fly in a Z car over a bimmer anyday.