Both of the cars you're looking at are known to BMW as having the E32 body, which are 7-series cars between 1988-1994 (USA). Other than the different engines and a few minor electrical changes, they are the same. These were BMW's top-of-the-line series cars so they're all "loaded" with only a few options. The options that I know of were the heated seats, a power rear sunshade, power rear seats, and maybe different sound systems. The Bentley's repair manual that I mentioned in my earlier post covers these differences.
My car is a '93 740iL (L is for long) so I'm familiar with its gas milage. Around town it gets around 18 mpg. On a trip from Florida to Savannah around Christmas it got 25-26 mpg. Also, the V8 REQUIRES premium fuel. I don't know about milage on the 735's 6-cylinder. I'd guess 20-22 around town and 27-29 highway and it can run on regular gas, but again, that's a guess.
IMPORTANT NOTE: A '93 740i is in the range of the infamous Nikasil problem caused by a reaction of the nickle in the engine and the sulfer in the fuel. My car has a Nikasil engine with 120,000 miles and I've had absolutely no problem with it. My opinion is that if were going to have a problem it would have shown up by now. Using high quality, name brand gasoline and synthetic oil is supposed to minimize the chance of having the problem. Also, the sulfer content of US fuels is much lower than it used to be. Still, here's a link if you want to read more about it:
http://www.koalamotorsport.com/tech/...shortblock.htm
Good luck.