I owned a 1986 E28 535i for about 10 years. Mechanically, it's the same car as the E24 635CSi, but the later is more desirable, as (probably) one of the most beautiful BMW designs ever.
Almost 1 year ago I sold my E28 535i and purchased an E86 Z4 M Coupe.
I suppose that probably my expectations related to the E86 were too high, therefore now I have mixed feelings about it.
Indeed, there are many reasons to like the Z4 Coupe. It's faster, quieter, more modern, sexier, steers better, has better fuel economy, has good value preserving potential for the future (but so does the 635CSi!), and so on.
However, there are quite a few of issues that I consider at least unpleasant (or even unacceptable) for a premium car built in 2006.
I liked better how the E28/E24 Getrag 260/6 gearbox shifted, compared to the ZF gearbox on the Z4M - not good for a 20+ years newer design. Then, there's quite a bit of "clunk" noise in the driveline on the 50.000+ Km Z4M, more than I had on the 200.000+ Km E28. The differential whines a bit when coasting from around 120 km/h down to 100 Km/h. I had to replace the front wheel bearings at only 50.000 Km. I also have a bit of steering wheel "wobble" around 120 to 150 Km/h whose cause I have yet to discover. You can also read a lot in this message board about failing rear springs (not happened to me, yet) and about electric assisted steering problems (only applicable to Z4 Coupe with the N52 engine).
Overall, I like my Z4M and I intend to keep it for long term. However, I want to point out that I miss my E28 535i - something that I wouldn't have believed it would happen, when I sold the E28 for buying the E86.
The M30 engine is extremely reliable. A reliability that you will not have with the N52 engine, so from this point of view you should better look for a Z4 M Coupe (S54 engine). Although not at all on par with the M30, I consider that the S54 engine is better in terms on reliability and servicability than the all-Al/Mg N52.
There was way more space in the E28, and the driving position was more comfortable than in the Z4, something that I miss during the long trips.
Servicing an E24/E28 is a dream, compared to E86. First, you can do yourself most of the work, without needing BMW special tools. Second, it is a lot cheaper.
If I were you, I would consider buying an E86, but I would not sell the E24 635CSi - unless it had a severe accident or it's badly rusted.