That's a tough call!

Here is my take:

If you prefer a car that might run forever I'd choose the 945T. That drivetrain is pretty tough. FYI - I drove my 90 240 to 300k then sold it. Original engine and drivetrain still going strong. The drivetrain is great but the downside is that most everything else wears out. I got tired of fixing the various leaks, brakes, exhaust (nearly constant), electricals (constant!) etc. Plus with the 945T the turbo will wear out.

If a fun and comfortable car are more important then I'd stick with the 855T. Accept the fact that it will cost more to maintain and enjoy it! Plus the 945T will be gone so you won't have 2 old WOWO's draining your funds. From what I've seen the interiors hold up better in the 8 series than the 9 series and 850's rarely rust. The 2/7/9 series cars will rust. The 855T could very well last as long as a RWD car with proper maintenance. I know of 850's going 400K+.

But you know your needs better than any of us...:)