[Flat Six]

I too had an '87 325is; loved that car. Never owned an ix but driven one and remember feeling a little unimpressed. Must've been the extra weight, but the ix just didn't have the oomph out of corners my younger self enjoyed.

Don't know much about the M10, but on the M20 make sure the head bolts have been upgraded (the original hex heads have been known to pop off the bolts at inopportune times); peek inside the oil filler cap -- if you see Torx head bolts you know they've been replaced. These cars are really easy to work on, too.

As with most cars of this vintage, condition is (nearly) everything. Looks like the 318 has some nice cosmetic upgrades; the seats in particular. And the ix interior looks spectacular.

If it were me, I'd choose the ix for 3 reasons: 1) it's stock/close to stock; 2) I love the sound of BMWs inline sixes; and 3) creature comforts [like power windows and better instrumentation] standard on the ix but optional/not available on the bargain-basement 318.

You also may want to reach out to T77911S here on PP -- he (and his son, I think) are really into E30s.