You question may come down to:
1. How much compression do you want?
If you are feeding a distortion, you may want to maintain a constant level so the sustain is as long as possible. If you are going on radio, they already compress everything. If you are live, how much dynamic range do you want to retain?
I'm not sure how much compression exactly, just a wide range, say from 2:1 - 20:1.
2. At what level do you want the compression to start?
If you are in a studio, you may want a line level compressor. If you are live, you may want something different for amp levels.
I'll be using the compressor to do home recordings with. I'll be driving the unit from my Focusrite audio interface, so line level.
3. Do you need variable controls for attack and decay time constants?
Some people want this to be fixed so they know exactly what they are working with and can't accidentally move an adjustment on stage. Some people want it to be variable so they can accommodate any playing style, such as in studio equipment.
Variable would be the ideal but I could live with fixed if I had to. It depends on the parts count and the cost involved. If I can buy a decent quad comp for significantly less that the cost of building one then I probably, reluctantly, will.
4. What do you want to use for a variable gain stage? FET? OTA? tubes?
FET's are easy to find but difficult to match with any consistency and this gets to be more of a problem with push-pull circuits that would be used in a studio setting.
OTA's are easier to use but more diffiicult to find.
My Hammond Solovox uses 6SK7 remote-cutoff tubes for its volume control and I did get a call one time from a guy on a fan forum of a band he was in to see if there were any 6386 remote-cutoff twin triodes in the US for his Fairchild 670 compressor. I gave him a few names of tube purveyors, but he couldn't find any. The 6386's are approaching unobtanium but a pair of variable 6SK7 / 6BA6 stages cascaded would probably give more compression with the added advantage that everybody and his dog carries them.
At the moment I'm open to all options, but as I said, cost is the main driver. After cost its about sonics and how suitable the circuit is to the purpose. I still have research to do on compressing drum overheads, for example (I'll be using the Glyn Johns drum mic technique). Once I know what the desirable compression characteristics are for this application I'll be able to narrow my search criteria. I realise that I may be on a fools errand and that I may be trying to force a square peg into a round hole but they're both risks I'm willing to take!
So you have some criteria here to select a compressor circuit. I hope this list helps clarify what you need to look for.
Thanks for your comments, they've certainly given me food for thought.