It depends on your needs. I am presuming that both are normally aspirated and are running about the same sales price ...

1990 was an exclusive year for a 740 - the front end setup (hood/headlight/trim) was one-year only deal, as was the instrument cluster, and perhaps a few other things too. The rest of the body remained pretty similar throughout the series, however. There are more creature comforts (power accessories), but more plastic trim inside to potentially break too.

The '83 242 would share a FI system with the '84, body parts from '79-'84, and trim from '81-'84. Though less luxurious, the 2-series is more durable inside.
The 242 has a classic styling that with a good paint job, they look younger than they are.

If you're single, the 242 looks more fitting IMHO. If you have a family or have to shuttle passengers often - get the 740.

Both can be equally reliable. Given that they've made it to 250k, the owners must have been doing something right. :)

-- Kane ... of course, there's always the option of buying both too!
--
Blossom II -'91 745Ti/M46 ... Bubbles -'74 144GL/BW35 ... Buttercup -'86 245GL/AW70
The Wayback Machine -'64 P220/M40 ... The ParaBox -'90 745GL/AW70