to answer your question quickly - they are pretty equal
1. the body is the same
2. the interior is the same, the '74 has different seats
3. the main difference is that the stock 2800cs has non-vented front discs and rear drum brakes. at this point in time, a good number of 2800cs have been converted to vented discs front + rear
4. there is an engine displacement difference, but power is basically the same on the stock motors. the 2800cs has higher compression and is perhaps revs more freely.
5. the 2800cs is often a lower price - for no explicable / real reason ... other than perception from the under-informed.
at this point in time, its all about how well the coupe has been cared for - rust avoided or remediated. most cars have been repainted by now ... so its all about the current condition. the best advice is to buy the best coupe you can afford ... the most expensive e9 is the one that you have to do a lot of rust remediation on, followed by repainting. interiors and trim are also expensive.