Dendrite is still in beta and isn’t feature-complete. I tried all three (Synapse, Dendrite and Conduit) and Conduit worked the best for me - I found it to be the most reliable and use the least amount of RAM. It also uses an embedded database (RocksDB) which makes setup a bit easier.
I tried joining several large Matrix rooms from my server, and the experience with Synapse was dreadful. It was using 100% of one core for long periods of time. In some cases it would just fall over and not join the room. Dendrite and Conduit are better in that regard.
Conduit’s weak point is its documentation. I had to read Synapse’s documentation to understand a few key concepts. I’ve been meaning to help write docs for Conduit but just haven’t had time. I’ve got a PR to improve the styling of the docs at least, but need to do some tweaks to it.