I'm in the process of starting a tank, so have no experience with either, but I'm also considering both. I spoke with the owner of Clearwater scrubbers a couple weeks ago to ask about ATS, and he made a couple of interesting comments worth sharing. One was that their recommended sizes of the ATS are often made for FWLR tanks, but if you're running a reef tank as I will be, he recommends I _undersize_ because the ATS can be so successful at nutrient export that it can starve corals from lack of available nitrates and phosphates. He even recommended I get their smallest model ATS for a 180G reef tank if I was going to run it 24hrs a day with a protein skimmer, and he's currently running a test of a system that completely replaces a protein skimmer with an ATS. Point being that scrubbers/reactors both seem to be extremely efficient at growing, especially vs a fuge.
Now as far as your actual question, the best pros/cons I've found between scrubbers and reactors is that some claim that ATS are more difficult to clean than reactors, so getting a scrubber with an easily removable screen can help. Probably the biggest reason I'd lean toward a scrubber would be because the chaeto that grows in a reactor is basically inedible, while the algae that falls off a scrubber can more likely feed your fish. On the other hand, if I had unlimited money, a big pro of going for a reactor is that then you can get the PaxBellum ARID which just looks really cool. If I get something, I'll probably get a Clearwater ATS, but I'll wait a couple of months as they are about to release another version that will be more easily cleanable.