Having asked this and seen it asked and argued many times, I wish to add my callow ideas.

I have a combo. 8" Sherwood.

These are maybe just things to watch for or think about when looking at what to buy.

Point 1. When you go from one config to the other, you (at least I) have to reset the planer tables as they do not lock back just so. This reintroduces snipe, curvature etc. The outfeed is the critical one. Unless you are lucky and or are really up with the settings, a few practice runs are needed to see if the setup is ok again.
I realise that planning will minimise this, but not all jobs are going to be that cut and dried in the home shop.

2. Again on changeover, the extractor system has two troubles.
- because it is flipped from one direction/end to the other, it can be a nuisance with the extractor hose.
- On my machine, if I have been thicknessing say 30mm timber, I have to wind the thick table down to about 130mm to clear the extractor unit.

3. When in thicknesser mode, the machine is a bit messy with wings everywhere.

4. Unless I have it all wrong, in thicknesser mode I am feeding wood in at one end, and the main on/off switch is over the other end of the side. I either have to walk around or lean over the machine to start it. Mine will allow me to stop it at the business end, but not restart. Other machines may well be better.

As I say, just thoughts and if anyone corrects me that's fine.